Beware the door not there before. You'll learn a lot of shorthand rules for dealing with the Weird safely (or rather, less wildly and randomly dangerous than it would be otherwise), and that one should be at the top of your list if you don't want to get lost in the worst possible definition of the word "lost."

  We use doors all the time. They get us through walls. They let us go in and out of places. And, the vast majority of the time, those places are well-known and physically contiguous between both sides of the door. Whether we regularly walk through a given door or not, we live with the confidence that it's supposed to be there, and that whatever is behind that door is also behind the wall that door is in. We know that the door was intentionally built into the wall.

  Beware the door not there before.

  There are always more doors than there should be at the Met Cloisters. And then there was one more. It wasn’t there yesterday, and it might have been there tomorrow.

  I aimed a masonry bit at the center of a carefully chalked X as my erstwhile assistant, a wizard-docent assigned to the Cloisters, fretted. There was already a small hole two centimeters directly below it, but the whirr of the drill still made him flinch as I created its sibling.

  The doorframe appeared to be at least two hundred years old, and it wasn't really masonry so much as a pile of thick, uneven stones barely ground flat on one side and held together with deceptively strong preindustrial concrete. The door it enclosed was an appropriately massive, dark slab of polished oak, accented by great black plates of wrought iron swirling around its edges. A huge handle, too long for me to fill with both hands on top of each other, curved from the right side of the door.

  "I thought you were going to use adhesive, Mr. Norton. I really should tell my superiors." Bernard, the wizard-docent, fretted.

  Bernard was a librarian, the broad term for low-talent or otherwise junior wizards in the New World Circle doing their time at academic and cultural institutions. There are plenty of magic users who can throw fireballs, or raise spirits or see through any of the many veils protecting the "normal" world from the Weird, but for each one of them there are six who can maybe do a potent working with several hours of prep time, several days of planning, and several years of practice. Those who haven't reached that point yet study and watch over the supernatural and supernatural-adjacent books and artifacts that fill every major research library and museum.

  Unless you have a specific academic reason to go through those "Employees Only" doors at the end of dark corridors, you only see the public stacks and galleries. Ever think about what they're really keeping under lock and key when King Tut's body, the Stone of Scone, and John Dee's manuscripts can all be seen either under glass or behind velvet ropes in the open?

  Okay, two out of three of those are fakes and the real ones aren't actually accessible to the public for good reason, but you understand what I'm saying. Just remember that some of the quiet, unassuming nerds poring over broken pottery in a closet or glaring at you from behind a tall desk might eventually be able to warp the laws of reality. Sometimes intentionally.

  For now, though, Bernard was personally non-magical and his nervousness was grating on me. I sighed and slowly pulled the still drill bit out of the hole, put the drill down, and picked up a can of compressed air to clear the dust out of it. He flinched at the loud hiss, like a cat.

  "I am using adhesive, for the doors themselves. You're going to be even less happy about it, because epoxy isn't nearly as neat as two small screwholes you can fill." Especially with how much I liked to use. I set the can down next to the drill and took a cream-colored rectangular box the size of a pack of gum from the open bag next to me. I waved it at Bernard. "I can use epoxy on the frame too, if you want a blotch of rock-hard goo and the sensor misaligned by up to three millimeters. Or I can sand down the frame until it's flat enough to use a removable adhesive strip with it. Which would you prefer?" The adhesive strip was a bluff. Do you know how weak their grip is on anything short of glass or tile in a climate-controlled environment with zero humidity?

  Bernard deflated. "I understand, Mr. Norton. I really do. It's just- you understand this is horrifying to me, correct?"

  I sighed. "I do understand, Bernard. But remember that it's much less horrifying than if I don't set up these sensors." I placed the box over the holes I drilled, confirmed they matched, and rooted through the bag for the appropriate-sized screws and anchors. "You know what happened in 1938."

  This historical reminder made him shudder. "I probably know more about it than you do, Mr. Norton. You're right."

  Some exposition is probably necessary at this point. Since I was on the same page as Bernard about the need of my work we weren't going to do it conversationally, and we certainly weren't going to talk about why I was called there in the first place.

  My name is Alex Norton, and I'm the paranormal technical support guy. Yes, the paranormal technical support guy in most cases, though there are plenty of aspiring knock-offs, along with some painfully bulky and inefficient workarounds certain secret organizations developed over the years.

  Magic and technology don't work very well together. They actually tend to wage religious crusades against each other. Technology is the application of science, in which the understanding of physical laws is used to develop mechanisms that depend on those laws being immutable in order to perform a task. Magic is the application of very different mechanisms that drastically change those immutable laws in order to perform a task. The reductive explanation is that intentional variations in the cognition and perception of an observer effect those changes, resulting in anomalous phenomena. The surprisingly less reductive explanation is that magic is a combination of focus, will, and some weird crap beyond what is able to be analyzed by the scientific method as we currently see it.

  I'm not a wizard, despite what the Circle might say. I'm a cable monkey who got caught up in the Weird after a very disturbing incident involving a warlock, a building's power system, and a lot of dead people. I was just a nerd in the IT department who caught on quick to some extremely bad strangeness just fast enough to save my own skin.

  Then I joined the Circle, became a librarian myself, obsessively researched exactly why and how magic and technology interfere with each other (determining that I had absolutely no talent for the former in the process), and kludged out a solution using three types of wire and the loosest concept of a Faraday cage. Then the Circle decided my invention was less useful in letting its members use computers and smartphones and more useful in upsetting the balance of power between some very big forces in the Weird, in the Circle's favor.

  I left the Circle, managed to wrangle a sort of intellectual property license agreement regarding Magebraid (since only one other person in their organization understood how it worked, and they're my best friend), and became an outside contractor. Which is why, when I'm not installing security sensors on doors that do not open into the space behind the wall in which they're set, I'm setting up liquid crystal smart glass windows for vampire crime lords and rigging remotes with copper and salt so ghosts can watch TV. Ghosts are so much less murdery if you give them something to do.

  The electric screwdriver in my hand stopped spinning and half of the sensor sat fixed on the stone doorframe. I pulled the other half, a larger, square box, out of my bag and set it on a sheet of newspaper next to it. Then I grabbed a fat syringe, a small plastic cup, and a wooden tongue depressor.

  "Where does this one go? No, wait, let me guess." I carefully mixed the two syrups from the syringe in the cup, activating the epoxy that would affix the sensor box to the door with the strength of concrete. "Big, old wood. Iron. Court territory?"

  Bernard looked at his clipboard. "It seems so. The wilds of the realm of the Fair Folk. Not one of the official paths, obviously."

  I spread the goo on the back of the box and aligned the gray strip on the side with the matching gray strip on the bolted-on rectangle. "And since it's iron, we're talking about either a very old route the Circle would rather the Court not know about, or a back way Nameless use for their penance."

  The docent coughed, "I couldn't speculate on that, Mr. Norton."

  "I wouldn't either, but if it's on the list where it's probably nothing too funky." The Cloisters has over a dozen-odd doors that by all rights shouldn't be there. Bernard's clipboard counted seventeen, but of those only nine had any notes of where they led. "Hold this, please? Right there, don't let it wiggle."

  Bernard pressed his hand against the square box, keeping it lined up with the rectangular one as I ripped a long strip of wide, black tape off of its roll. I carefully put several inches on the wood above the sensor, then steadily inched it over the box, removing every bit of slack I could as it passed over Bernard's receding fingers. I checked the alignment one more time, then held the other end of the strip against the wood below the box, making sure it was tight and wouldn't shift while the epoxy set.

  I brushed my hands, wrapped the used epoxy cup and stick in the newspaper, capped the syringe, and packed all of my tools back into the bag. Power and wiring would be for tomorrow, when all of the sensors I couldn't screw in place would be (mostly) permanently set in clear resin. "That's twelve, right? Five left? We're making good time, Bernard."

  He checked his clipboard again and nodded. "Yes. The next one on the list is by the Merode room." He squinted at the note next to it. "It opens onto a street in Jagged Flats."

  I clucked my tongue. Jagged Flats is a small desert town next to the hole in the world. It's sort-of in Utah, but because the hole is metaphysically literal and space gets very strange around it, the actual geography gets confusing. Getting there is also awkward, unless you're willing to ride a subway that can possibly erase you from the Earth or drive in several arbitrary directions for an arbitrary amount of time while being very careful to never look directly in the rear-view mirror. A door there would be very convenient. "A street you say? Does it say which one?"

  Bernard shook his head. "The indefinite article. And the "a" is underlined."

  What a shame. "Tell me it's triple-bolted."

  "Of course."

  I slung my gear bag over my shoulder and nodded towards a flight of stone stairs. "That way to the Merode room, right?"

  ***

  The Merode Room at the Met Cloisters is, like most of the complex's other galleries, a little chunk of medieval Western Europe on the north tip of Manhattan. Dark wood floors and equally dark wood ceiling, wide arched windows with shutters to match, flat walls of aged white stone, a fifteenth-century triptych on the mantle, nothing really that interesting. Note: I'm not a historian and the triptych is just a piece of art and not a strange artifact or depiction of a prophecy, considering how ambivalent Bernard seemed when we entered the room (I could tell how potentially dangerous or thaumatergically significant any artifact in Cloisters was by how hard he seemed to clench when I walked by it with my tool bag swinging).

  Two archways lead into the Merode Room from where they should: one from the Boppard Room and one from the Late Gothic Hall. I checked the Cloisters' floor layout before I packed my gear, so I immediately clocked the nondescript door to the right of the Annunciation Triptych. It was as plain and unassuming could be, a flat slab of light, varnished pine recessed into a beveled, off-white frame. A gray doorknob, just as bland as the door itself, stuck out of the right side.

  "This one, right? Simple, though I'll have to lift the sensor block a bit." I dropped my gear bag in front of the mantel holding the six-hundred-year-old work of art with a clunk.

  Bernard's frown went beyond the nervous disapproval I had gotten used to and his eyes jumped between his clipboard and the door. "That... one's... new." He turned and pointed at the opposite corner, next to the entrance to the Late Gothic Hall. There was a second door, more congruous to the rest of the room, made of vertical wood beams bound with two wide iron strips. Another beam, hastily hewn and artificially worn, lay across the doorway at waist height. Three thick padlocks, seemingly from three different centuries, secured it to industrial steel brackets on either side of the door. "That's the door to Jagged Flats."

  Oh, crap.

  I told you, beware the door not there before. And the more "not there before" the door is, the more you need to beware it.

  Bernard and I approached the mysterious new door and examined it. It seemed utterly plain, like an office hallway door, or the door to an apartment bathroom. It had no keyhole, and the doorknob was perfectly round and smooth. We knew better than to test it.

  "This isn't a supplies closet you just never noticed before, is it?"

  He shook his head and swallowed. "It isn't."

  Like I said, oh crap.

  I stepped back and picked up my bag, grabbing a roll of yellow security tape. "I'm not touching that until it's mapped in some way." I slapped the end of the tape high and several inches to the left of the door, precariously close to the triptych. "Until then, we tape it off. Call Staten Island and get a team in." I drew the tape down to the other side of the door, affixing it to the wall at ankle height. I repeated this lower-left to upper-right, marking the door with striped yellow plastic that told any visitors to not, under any circumstances, open the door.

  There weren't supposed to be any visitors for the week; officially, the Cloisters were closed for annual maintenance. That meant installing the new sensors on the wrong (in the "should they be here?" sense) doors, then giving the Magebraid insulation on the information kiosks and point-of-sale systems a once-over to make sure they won't crash or get bricked by a post-librarian wizard researcher or supernaturally charged tourist. I was being overly cautious with the tape; you would be, too, if you knew where some doors led.

  Bernard already had his phone in his pen hand. It was an inexpensive Motorola, covered in carefully arranged patterns of braided, glued, and lacquered wire that identified him as a happy customer. "Of course. I'll request a team from High Rock to investigate this immediately."

  "I would strongly request otherwise." We both jumped at the voice.

  A visitor stood in the entryway from the Boppard Room. Like two of the doors, he wasn't supposed to be there. He was a spry man somewhere between the age of fifty and seventy depending on exactly where on his face you looked. Short white hair, a trimmed white beard and mustache, light blue eyes accented by laugh lines. I recognized him immediately, even if the photographer's vest and brown fedora (which he could wear without looking like he was about to correct someone on a subject he knew nothing about) wasn't his usual neat linen suit.

  Oh, crap. But in the more playful tone of someone getting ready to be head-smashingly confused and irritated and not exposed to the unspeakable horrors of a heretofore unknown realm and the utterly alien monsters therein.

  I sighed. "Hi, Grandpa. It's that bad?"

  Grandpa Paradox, or as he'd prefer, Dr. Gustav Polchin, was and is a theoretical physicist. He was, in the sense that he studied theoretical physics. He is, in the sense that he's a physicist whose existence is varying degrees of theoretical. I'd call him a time traveler, but he's more of a time wanderer, or a time retiree gardener, or a time pacer-around-the-room. He was/is/probably was going to have been deeply confusing. I'd call him a friend.

  "Who are you? I-" Bernard's voice raised, then froze. Or rather, was frozen, as if time stopped for him.

  Grandpa stepped towards him, eyes focused on the poor docent. "Yesterday Magus Albright asked you to come into High Rock today to prepare some abstracts for an ongoing project. He assured you that Mr. Norton could complete the installation on his own, and implied that he might prefer not having you look over his shoulder."

  I shook my clipboard at Grandpa, the only other person in the room. "I assume this is important. This job has been enough of a pain in the ass on my own, and now I've got this-" I trailed off, glanced at the place where Bernard wasn't, and winced at a hangover-like wave of headache and nausea that washed over me and scattered just as abruptly. "You know how disorienting that is! Did you say Albright called him in?"

  He shrugged, seeming oddly stiffer, less affable than he usually was. "He did after I call him. It's on my list. But first I should introduce myself, Mr. Norton. I'm-"

  I rubbed the bridge of my nose. "I know, Dr. Polchin. We've already met."

  He looked at me oddly, cocking a carefully trimmed white eyebrow carefully. It usually was bushier. "I met you tomorrow, Mr. Norton. You're meeting me today."

  Pure palm to face. "No, I met you three years ago. A week after that you met me."

  Grandpa Paradox rolled his eyes. "You're much slower on the uptake than you were tomorrow, Alex. I am Dr. Gustav Polchin, but not your Dr. Gustav Polchin."

  I pulled my hand from my face and stared at him with a look of horror, the kind you get when your uncle tells you he wants you to show him how to build a web site. "Parallel universe."

  "There we go, my boy. And I'm here to help." He smiled thinly.

  The plain door next to the mantel loomed behind its caution tape. I nodded towards it. "With that? Kind of a long trip when we have our own you, isn't it?"

  The alternate Grandpa Paradox gestured to the archway leading outside, "I'd rather not stay in the presence of that thing for too long. Let's get some distance first."

  We walked out to the cloister proper, a pleasant little quadrangle of garden with perpendicular paths intersecting at a stone fountain. "Temporal stuff is enough of a pain when I'm around the local you, Gramps. Adding any sort of multiverse to the mix is going to rapidly feel redundant."

  He produced a pack of cigarettes from his vest and plucked a filterless stick from it before offering one to me. I shook my head. He shrugged and struck a match tucked between his fingers, mumbling through his teeth as he lit up, "It's safer and more simple, actually. Like you said, my counterpart makes things complicated in the sense of perception and time."

  "And you don't?"

  He snorted, shaking the match out and flicking it into the grass. "I'm new here. Less of a trail, less of a presence. Your Gustav Polchin has a much larger nonlinear profile that would be ill-advised to expose to that particular door. So he is performing this role in my world. Our shoes are cleaner that way."

  I considered just how disorienting Grandpa Paradox usually was when he dropped by. Even setting him up with a phone was a confusing ordeal that left me staying in a hotel in Hoboken for a week just so I wouldn't meet myself. "Makes sense. So, are you from an alternate timeline? Does that door lead to some kind of wacky shenanigans there you want to seal at both ends?"

  Other Grandpa Paradox laughed, a mirthless bark through his burning cigarette. "I wish, son. What do you know about transaxial meta-topography?"

  Ugh, technobabble. Don't get me wrong, I like technobabble when it's my kind of babble. Going into crazy super-science? Not so much. "Comics, video games, arguments about canon, the obligatory Star Trek reference."

  He sighed wearily. "And you seemed so clever when I met you tomorrow. Very well. I'll try to keep it simple. Alternate timelines, multiple dimensions, parallel realities, they can all be mapped like any physical or temporal direction. They're axes, just like X, Y, Z, and delta."

  I nodded impatiently. "Yeah, yeah, and there's a fifth dimension, sixth, seventh, so there are multiple realities. Like I said, I read comics."

  A spiteful cloud of smoke was blown in my face. "Don't interrupt me. There are additional axes, and everything that exists has coordinates in those axes. Picture a location on a map, with a location of X and Y. Add the third dimension, Z, and you can have a point stacked on another point but still the same location on the map. Add the fourth, delta, and you have a point that can be in the same location in three-dimensional space, but occupying a different place in time. Now you have two separate vectors by which a point can be distant from another point but still have the same coordinates on the map."

  Okay, that’s all bullshit I made up. It’s the best I can wrap my head around higher dimensions. Look, circuit diagrams are fine for me, but when you start to discuss manifold topology my poor nerd brain shuts off. And Dr. Polchin stopped trying to explain it and just glared at me when I started giggling at the term “D-brane.”

  From what I could understand, Grandpa Paradox moved up and down time, and sideways in time, which I was already half-used to. And Other Grandpa Paradox was from a parallel reality not related to time, so he was from further away than any place our Dr. Polchin would ordinarily be.

  I looked over my shoulder towards the Merode Room. "And you're here because that door goes somewhere even further away."

  "Further away, yes. Along more directions than you’re willing to count. Far enough that the laws of physics change significantly." I glimpsed what might be considered fear in his eyes.

  That was worrying, but it still didn't sound that dire. "Different laws of physics are old hat. I know some of the doors here go places where gravity and nuclear bonds get wonky. Even the Court's realm turns linguistics into energy, in its own way."

  "The question is what kinds of things change, and that's the concern. A place that… far away, that's where metanarrative echoes can appear." He reached into a large pocket on his vest and produced a paperback I immediately recognized.

  The cover showed a man in a cubicle, with tentacles sprouting out of the next cubicle and a mushroom cloud in the background. One corner had a familiar dent. "Hey, that's mine!" Then I noticed the black swipe of a marker across the top edge and became really indignant. "You wrote on it?"

  Other Grandpa Paradox rubbed his temples. "Please try to focus. Unwanted door to a very, very far place that is very much like that." He pointed at the novel.

  I bit my lip. I love the series. It's very much about eldritch horror, the scope of which is probably a fair bit larger than the singular entity whose incursion and subsequent explosion is responsible for the sleepy and terrifying desert town of Jagged Flats. It also describes a thaumaturgical landscape where getting the attention of those entities is much, much easier than it is here (Jagged Flats happened as a result of the American government's magical counterparts to Operations Paperclip and Crossroads; a similar thing can happen over there with a Spirograph and a T-square).

  "Point made. So the 'there' behind that door is actually... there?"

  He grunted, "Or close enough that the rules are effectively the same. And a connection that distant can cause those rules and ours to bleed together. If anyone opens that door..."

  My phone was already in my hand. "I get it. I get it. I wouldn't trust the Circle to be smart with it, either. I'll put out a GAEA call, or maybe see if NASTY has some boffins that can help-"

  Dr. Polchin immediately cut me off. "There isn't time for that! If that door isn't sealed by tomorrow, it will open!"

  Wow. I didn't think I had ever heard Grandpa Paradox say there wasn't enough time for something without winking and making a smartass quip. Or correcting someone else saying there wasn't enough time by winking and making a smartass quip.

  I stared at him. "Okay, okay, we can't wait for the network to page somebody relevant and the squaddies are based out of Illinois and that's too far. Rush job, I get it."

  The good doctor watched me impatiently, but gears were already turning in my head. "I think I have a plan, or at least a pen to start writing one down." I called said pen.

  I heard three rings, then a voice much more chipper than most of the greybeards at High Rock. "Yodel, Alex! What can I do for you?"

  He was in today. I pumped my fist against my chest. "Hey, Gary. got a big favor to ask. How fast can you get to the Cloisters?"

  "I'm doing some plotting right now, but maybe this afternoon?" Plotting could have meant solving math problems or forming sinister (for a complete nerd) schemes. It didn't matter.

  I was already walking towards the exit of the complex. "That shouldn't have been a question. I need you here, now. Get here as soon as you can."

  There was a pause. "If it's that important, I can get a team up there. Is this about the doors?"

  "No team. I need your expertise and I need the Circle to not be aware of this for the next day or two. Get a car, I'll pay you back." I rolled my eyes. "If you can make it here in the next hour, I promise I'll work on modding a CX for you."

  Gary's squeal stabbed into my ears. "Color screen, Python programmable?"

  And a lot more difficult to insulate with Magebraid than a TI-89. The smaller and more complex the electronics are, the harder it is to chart exactly how energies need to disperse around the components. That's why I kept putting off doing it. It's also why I charge more for phones than laptops. "Just get up here."

  ***

  Parking's a pain in the city, but it's better than schlepping your gear through the subway. It's also better to have a piece of equipment and not need it than need it and not have it, which is why the trunk and back seat (basically the same space; such is the glory of a utility hatchback) of my beloved boxy dorkwagon was filled with tools and electronics components of all types. I rooted around in the carefully constructed mobile junk pile, picking out a few useful items for what I had in mind.

  "Come on, I know you're in here- ah!" I pulled an ugly device a little larger than an old-fashioned telephone handset out from under the passenger seat and tossed it in the plastic crate I set next to the car. It settled with a plastic clunk next to, appropriately enougn, an old-fashioned telephone handset. Both sat in a comfortable nest of various types of coiled wires, framing some alligator clips, copper strips, and a soldering iron like two birds guarding their eggs.

  Dr. Polchin was markedly unhelpful as I put together this ad-libbed kit. He hung back, leaning against a wall, seven or eight cigarettes into a pack that never seemed to shrink. I glared at him as I closed the hatch on the beloved and battered Scion xB and hefted up the crate. "You know, my Grandpa never smoked."

  He smirked and snorted, flicking another smoldering cigarette. "I assure you, my boy, my counterpart is almost certainly acting exactly the same as I am, back home. Indulge an old man in his reactions to stress."

  I couldn't imagine Grandpa Paradox stressed. He was always a friendly, affable, and downright languid man, popping up and disappearing as he pleased. The Doctor by way of Tom Bombadil. Even the few times he showed up to warn about some impending non-linear doom to be untangled from the thread of time, he'd nudge me with a wink, a grin, and a cryptic hint. But stressed?

  The implications were terrifying. I'd have almost preferred a cryptic hint to him showing up and directly pointing at a Very Bad Thing and telling me We Should Both Be Afraid. I tried to push those implications out of my head and focused on the lack of a cryptic hint.

  "You're also much more coherent than I'm used to. I can actually follow pretty much everything you're saying, and it sounds like it should be more complex than the usual time wonkery he'd put me through. It's downright weird having a direct conversation with you."

  Not Grandpa sighed. "That's why I'm here, son. On both counts. He's walked up and down this particular bundle of timelines for literal ages. This world's new ground to me, so I need to keep things relatively simple. For me to navigate, for you to understand, and for that door to not shake from any fifth-dimensional stomping around."

  Did that make sense? It seemed to make sense. "Ah."

  The old man patted his vest, then pulled two pocketwatches out a pocket. One was gold and one was silver, and which was which seemed to fluctuate whenever I focused my eyes on them. "Speaking of which, I need to get my distance from here, and drop by that fancy little magician club of yours yesterday before I forget about it."

  I deflated. "That's it?"

  "I told you all I can, son. The rest, as they say, is up to you." He dropped both watches back in his pocket. He turned away, then paused and turned back to me. "One thing to remember. When there's knocking on the door, turn the knob."

  And just like that, I was alone in the parking lot, my arms getting sore from their payload of junk.

  "What?" That was more like the Grandpa Paradox I was used to.

  ***

  Gary Sherman was a stocky but spry man, a good foot shorter than me, with buzzed gray hair and a round face. He was cheery, with a high voice and an odd bounce in his step. He wore a flowing velour robe and a floppy pointed hat, both royal purple with glittering silver stars embroidered on them. A large shoulder bag made of the same purple velour swung heavily against his chest and side. Seeing him emerge from an ancient burgundy Dodge Omni was its own surreal experience.

  "Jesus Christ, Gary." I stared, agape.

  He grinned, "What? You need a mathemagician, so here I am!"

  In the Circle's numerology research department at High Rock Castle, Gary was the only person willing to call himself a mathemagician. I mean, he was, very literally, but the rest of the equation-shuffling wizards there didn't appreciate the title. They also tended to dress more staid, either in your standard business casual or the sort of neutral-color wizard robes you could picture sourced from the same uniform suppliers that produce pharmacists' lab coats. "Were you actually working in that?"

  "Oh, no, I changed in the car. They hate it when I wear my fancy hat." He pointed back at the car, "If this is too much, I have a fursuit-"

  I jumped. "No, no! The mathemagician in his robes of office is all I need!" The furry thing wasn't a surprise or a problem in itself, but it's very hard to type or pour salt in big costume paws with no thumbs.

  Gary bowed with a flourish. "I'm happy to help, especially with the promise of a new, superior calculator. So what do you need that's so urgent?"

  I nodded towards the entrance. "There's a door that shouldn't be here."

  He furrowed his brow, "There are lots of doors that shouldn't be here, Alex. That's the point of the Cloisters. It's the House of Doors!"

  That was true. The Cloisters was built at an intersection of leylines specifically to provide a place where passages here to elsewhere could manifest with some consistency. It's basically a grommet to let a bunch of strings run through a piece of fabric instead of putting a whole lot of needles through all over it. Doors that appear in the Cloisters don't appear randomly in alleys, where any random person could be drawn to them.

  You might think making such a place a public museum annex would be self-defeating, but that's actually a big reason it works. The flow of bystanders can be controlled and monitored here, and it's much easier to keep them away from doors they shouldn't enter when those doors can be kept locked and marked as employees-only. The latter is actually much more effective than the former; the assumption that a door will open into a boring closet or hallway as good as a stealth enchantment.

  It helps that having hundreds of people completely ignorant of the Weird pass through every day disperses a lot of the energies that are drawn there to spontaneously form doors. It's a heat sink of liminality.

  That doesn't make the doors there any less dangerous, any more than a heat sink means it's safe to lick the RAM socket next to it. That’s why I was putting sensors on them, so the right people could be alerted if any of them open.

  "This one's apparently a lot worse. A Grandpa Paradox came by to warn me about it."

  Gary whistled. "Dr. Polchin, eh? I guess it's a door to another time? He warn you not to save Kennedy?"

  I led him through the entrance hall towards the garden. "A Grandpa Paradox, not our Grandpa Paradox. Parallel universe instead of alternate timeline, apparently."

  If you spot this narrative on Amazon, know that it has been stolen. Report the violation.

  "Ooh, neat!" He skipped as he followed me. "I haven't seen that before. Not counting dimensionally adjacent realms, of course, but who does? Where is he? Or should I say... when is he?"

  Ugh. I have two personal rules regarding anything fourth-dimensional. One: Never say "or should I say, somewhen?" Two: Never complain about time travel giving you a headache. But it was Gary. It was to be expected.

  "Yesterday, probably. He only came by to warn me about this new door." I thought for a moment. "He said it involved, uh, 'transaxial meta-topography.' And something about branes"

  Gary nodded, "Ooh, that would indicate (something about bosonic string theory and vector space and something called Calabi-Yau which I thought was a Warhammer 40K faction). Again, I say: neat."

  I blinked my eyes back into focus, then nodded curtly. “He also talked about D-branes.”

  “Well, that’s part of it, of course. Heh, D-branes.” We both snickered.

  I pointed towards the Merode Room. "There's a door there that apparently goes somewhere, um, far enough away that the physics start to get fictional." I showed him the marker-vandalized novel. "He said it goes here. You've read it too, right?"

  I swear huge four-pointed stars gleamed in Gary's eyes, like an anime character being told about treasure. "Magic defined entirely by geometry and programming! A world of thaumaturgy without any focal subjectivity! Pure math as pure magic!" He ejaculated, rhetorically.

  This reaction was expected. I simply stared at him for a few moments and waited for the other shoe to drop in his mind.

  The excited smile on Gary's face faded. "Oh, no. That's terrifying."

  I choose my friends carefully, and the attribute the most is that their character and intelligence always outweigh their ambition. I know terrifyingly powerful entities, rich people, and guys who can simply kick my ass, and the thing that they all have in common is that, given the opportunity to rule the world, they'll write it off as being too much of a pain in the ass to bother. It helps that most of them aren't dicks, either.

  "Yup. Which is why I called you here, bud." I pointed at his bag. "Your laptop has Mathematica on it, right?"

  He snorted, "Does the Cox-Zucker machine always make me giggle?"

  I led Gary into the Merode room, where the door was still taped-off and closed. "Okay, get settled and I'll put something today."

  He pointed at the offending ingress. "That's it, huh? Endless unspeakable horrors and reality-rending catastrophe behind it?"

  "Yeah, but this one's worse than usual." I gestured at the bolted door on the opposite wall. "At least with these we understand the rules."

  Gary was already sitting in a purple velour heap in the middle of the room, his computer on his lap. It was a chunky black Thinkpad, the kind offices would issue to staff a decade ago, covered in an uneven grid of tri-color wires "Speaking of which, what exactly is the plan here? The whole thing about the rules being completely different and unfamiliar, makes me think you're shooting from the hip."

  I settled on my knees, picking through the boxes I brought in from my car. "I am, just with a soldering gun." I unspooled an orange extension cord and walked the plug to the outlet between the archway leading into the Late Gothic Hall and the bolted-shut portal into a pleasant town on the edge of the yawning maw of reality (the less dangerous one in the room). I plugged a power strip into the other end and set it in front of Gary. "Make sure it's charged while you get things ready. We'll draw the circle once everything's set up."

  The screen on his laptop was already open to the inscrutable-to-me home screen of Mathematica 8. He drummed his hands on the computer's hard plastic wrist rest. "And what am I getting ready exactly?"

  Carefully sorted piles formed around me as I emptied the boxes. I pulled an ugly-looking black gun from its case and plugged its cable into the power strip. I pulled the trigger and grinned at Gary as the gun gave a quiet hum and the thick, square wire loop on its muzzle started to glow red. "The worst math you've ever done."

  He stared blankly at me. Yeah, that was fair. I took my finger off of the trigger and set the gun carefully on a wire stand before picking up the paperback from one of my files and tossed it to him. "Off the top of your head, how many technical, pseudo-technical, and pseudo-supernatural-technical terms does he regularly use?"

  "Many."

  I nodded. "Right. And how many are actual terms in geometry, physics, or magic?"

  Gary squinted and cocked his head, as if counting to himself. "Um, some. Not many."

  "Again, right. A lot of, for here at least, completely made-up terms. But there are still some actual principles, purely from mathematical and scientific perspectives. Take away the series' conception of magic and you have at least a handful of things you're familiar with. Things you can punch into that laptop and plot curves and grids and contours that make sense to you, right?"

  He shrugged, "I mean, sure, but that doesn't mean anything. I practice invocation through the abstraction of scalar variables and vector calculations, using the differential of perceived scope against absolute values as a cognitive catalyst. What we're talking about is algorithmic plotting of complex geometric formulae combined with the Enochian equivalent of a low-level programming language."

  "You use psychedelic calculus and they use Cthulhu's sacred geometry. I know, we're on the same page." I might not understand complex mathematics, but if you hum a few bars...

  Gary poked at his keyboard, then opened the paperback to a random page. Then he put the book down again. "Alexander?"

  "Yes, Gary?"

  "Are you asking me to do math that will invite dead alien gods from a distant reality to eat my brain?" He crossed his arms and I stifled a laugh. The situation was dire in theory, but he was still a sulking wizard wearing sparkly purple robes and a pointy hat.

  The reaction was once again expected, if a little late. I grinned again and shook my head slowly. "Just the opposite. I'm asking you to do that math as wrong as you possibly can." I held a brick-sized plastic enclosure in my hand and pressed the soldering gun tip to it, melting the material and cutting a rough circular hole in it.

  Don't judge me. I'd measure and drill and cut, but this was a rush job and I was flying by the seat of my pants. I could put it together neatly and without fumes, or I could make the components fit together before the door that must not be opened started to open.

  Gary eyed the smoking box as I burned another hole into it. "No offense, but if you wanted bad math you could just do it yourself.

  I mean, he wasn't wrong. I threaded some wires through a perforated circuit board. "You know what actual math is being referenced and what is unspecified weirdness from beyond the stars. I don't. You also know how to use Mathematica, and the last time I touched it to do anything besides install it for you was high school. So I want you to take the concepts you recognize from the book and do them wrong. Can you do that for me?"

  We are capable of expressing many emotions through our faces. In fact, we can convey more emotions with our expressions than we have words for. Gary didn't look at me with disgust, or horror, or bemusement. He looked at me with that precise facial expression given when a friend blows a huge cloud of weed smoke into your cat's face, and the cat clearly doesn't care and is already loopy on catnip, but you still sigh and say "Don't do that to my cat."

  Gary sighed. "I'll see what I can do."

  ***

  "Any voices in your head, Gary?"

  "No."

  Five minutes.

  "Any voices in your head?"

  "No."

  Five more minutes.

  "Any voices in your head?"

  "No."

  Five more minutes.

  "Any voices?"

  "No!"

  Five more minutes.

  "Any voice-"

  "The Sleeper awakens! It is the Muad-dib of Slaanesh and his thousand goats! Lo, the King in Yellow calls from his house on the borderland and so shall the song of his daughter Saya be sung and the halls of Ravenloft shall open!"

  Five seconds.

  "Okay, I'll stop asking. Just making sure."

  ***

  An hour and a half later, the door was still mercifully still and silent, and I was wrapping up some very sloppy scripting for an Arduino very sloppily soldered into an acoustic coupler. Said coupler was itself very sloppily soldered into a caseless tube amplifier, a mishmash of raw components set into a naked circuit board.

  Basically, I had just finished building the world's worst DAC, complete with capacitors that would become near-lethal to touch, assuming the entire thing wouldn't catch on fire and short out the power strip once I connected it through the 1,200-watt PC power supply feeding the device.

  My craftsmanship is for when I'm in the workshop and have a decent bench and more than a car trunk's worth of parts and tools. Outside of that situation, I'm the king of kludge.

  "You doing okay, Gary?" I heard him groan next to me.

  He closed his laptop, then his eyes. "Okay, some good news, some bad news."

  I mentally flipped a coin. "Good news first."

  The mathemagician opened his eyes and looked at me. They were red, which was much more encouraging than green and glowing. "Considering just how limited my references are, I think I did a pretty good job. I'm working off of memory for the rest of the series, but the first book lays the groundwork with two central mechanisms, a curve and a grid. Those are the building blocks, along with the idea that algorithmic calculations can automate magic. Ridiculous here, catastrophic there."

  "But you think you did a good job?" I looked down at his closed laptop.

  Gary threw his hands in the air. "The curve I pulled out of my ass and plotted the midpoints of the golden and silver spirals. I figured it would be a good starting point. The grid is usually described as 'a pentacle, but with weird bits,' so I just came up with some visualizations of a pentacle with weird bits. Then I asked myself how you would try to do it, and began adding random coefficients while picturing myself mashing my face against the keyboard while shouting 'I can do math!'"

  I folded my arms, but that was a pretty accurate dramatization of how I would tackle higher mathematics. "So, good news. Now the bad?"

  He tapped the top of his laptop absently, staring at the floor. "I'm worried that trying to botch these numbers could have the opposite effect. Isn't part of the premise that guys like me can fall ass-backwards into summoning horrors?"

  Uh-oh. Still, he was being speculative. "That sounds like more of a worry than bad news."

  Gary slowly opened his laptop again. The lock screen popped on. "I think I might be starting to Kafka, Alex. Some of these visualizations feel like I'm looking into an abyss."

  Another, louder uh-oh. "You said you didn't hear any voices, Gary!"

  "I didn't!" He whimpered. "I told you, it's like staring into an abyss. A great yawning nothingness under the skin of the universe that-"

  It was time for a distraction, and unlike my friends I sometimes am a dick. "Nietzsche."

  Gary looked sharply up at me. "What?"

  "Nietzsche was the abyss staring and god being dead. Kafka was the big bug and Cardassian legal system." I sat next to him, "But that's good! The abyss thing is good! Kind of."

  He stared at the lock screen on his computer as if afraid of what was behind it. "My magic works by conceptualizing values that aren't consciously conceivable, and I haven't had an attack of existential horror because of it since I was fifteen. Looking at some of the visualizations I'm making feels like I'm standing on a cliff in Utah."

  I squeezed his shoulder. "Because there's nothing there. That tells us two things. One, that the purely Platonic metaphysics of the world on the other side of that door are starting to bleed through already, which means the fire Gramps lit under our ass is very justified."

  Gary whimpered again. "How is that good?"

  "Because it's a void. You're crunching numbers that can wedge a crack open in reality, that's a big concern, but there isn't anything there right now. The rules are shifting around here, but nothing has actually come over yet. Or is aware of us."

  I clicked the button below the trackpad on Gary's laptop. He winced and looked away.

  The screen showed four static charts of spirals and ellipses and polygons and lines. They looked vaguely disorienting, but that was only because my eyes kept trying to follow the odd curves one by one like I was decoding an optical illusion. The conceptual significance of any of it was entirely lost on me.

  If a diagram's more complex than a waveform, you're probably going to have to walk me through it. That was exactly what I asked Gary to do.

  "Explain it?" He had tossed his hat behind him and was wiping his sleeve on his forehead. "Alex, just trying to comprehend it is giving me some very worrying near-hallucinations."

  I carefully turned the laptop away from him and pulled it into my lap. "And that's why you're going to take your mind off of it, because you're too good at this. It's why I love you, bud." I squinted and poked around the screen, trying to figure out which fields I could modify and then vandalize. I wasn't exactly familiar with Mathematica.

  Gary swallowed. "I'm going to get taken over by an extradimensional parasite, aren't I? Or my brain's going to melt because of this."

  Tap. Tap. Tap. Move a decimal point left here. Increase an integer by one there. Transpose a variable over there. It took a few times for my adjustments to not make Mathematica vomit error messages at me, but eventually I replaced the charts that had scared Gary with new ones. Some were only slightly different, some wildly so, and all were significantly uglier. "You did the hard part for me here. Give your brain a rest and go over to my laptop, double-check that the Arduino script will give a proper modem handshake? But first..." I spun his computer back around to him. "Tell me what any of this means."

  First he cringed away from it. Then he blinked and stared at the charts. "It's complete nonsense." His eyes widened, "Dammit, I didn't save my work! You just crapped all over an hour and a half of progress!"

  I swear, some academics. If he was a geologist, he would have been angry at me for pulling him away from the mouth of a volcano because he was really excited about the seismic readings he was going to get falling into it. Or something. I'm even less of a geologist than I am a mathematician.

  "First the math scared you, and now it pisses you off. Gary, this is the progress we need." I pointed at my own computer and the worryingly thrown-together DAC and amp next to it. "Check the handshake script, then take a break. We're almost ready. Oh, and toss me that PC card modem from the pile on the right."

  The long-suffering purple wizard crawled towards my own messy work area on the floor. "You sound like you have a plan. I'm trusting you to have a plan."

  "Of course I do!" I gave him the widest, most knowing grin I could, considering just how wild a swing I was setting up. "We're dealing with a universe where being blinding with brilliance is the most dangerous thing you can do."

  Gary was already bent over my laptop, frowning at my code and trying his best not to focus on the horrifying electric chimera sitting on the floor in front of him. "And?"

  "And we're going to be more baffling with more bullshit than humanly possible."

  ***

  I'm not a mathematician. I'm not a programmer (my Perl and Python coding has been described by several colleagues as "horrifying" and "(unintelligible retching)". I'm not a physicist, or a wizard. I'm a tech support guy, which means it's my job to not only fix things, but to figure out how to fix things on the fly.

  Being able to figure out how to fix things means first being able to figure out how things break. Not destroy, but break.

  A mechanism working and a mechanism not working go hand-in-hand, because in both cases the mechanism is there. It's the yin and yang of engineering. Short out a component on a computer, and you can identify it and replace it, making the computer work again. Throw that computer into an industrial shredder, and it simply won't be a computer anymore.

  What's what Gary was for. To keep anything from opening the door or going through it, we needed to present it with computations alien enough for them to be wrong, but not enough for them to not be functional math at all. It's the difference between speaking in baby gibberish and in communicating almost entirely in malapropisms and bad grammar.

  Basically, my plan was to irritate if not outright offend the eldritch beings on the other side of the door with such a bastardized nonsense version of the pidgin cosmic language the humans of that world used to (very hazardously) communicate with them that they would, hopefully, give up and leave us alone for being utterly compatible. If this Platonic universe and its mind-eating horrors wanted to share a wall with us, they were going to hear our speakers through that wall as we sang along with Eurovision's top hits, translated into English, then Enochian. Through Google.

  If it was complete gibberish and barely different from background noise to the entities and forces over there, they might still see us as fertile grazing grounds with just enough consciousness (and almost no defenses) to feed on.

  If it was something they kind of understood, but was fundamentally wrong in some very stupid ways? Played loudly through the walls through an ugly and dangerous homemade amp?

  Well, that's how you get the neighbors to move, isn't it?

  ***

  Gary had shed his comical velour robes and left them in a pile next to his hat, and was simply wearing plain black jeans and a beaten-up Bad Brains t-shirt. He stood over his laptop, fully charged and disconnected from the power strip, staying still as he gripped one end of a string.

  I held the other end, and we kept it taut as I paced around him. My other hand held a large container of kosher salt, keeping it carefully tipped to pour it consistently in a circle around Gary and his computer. I closed the circle, the canister still half-full (though two other empty ones sat by the archway to the garden, their contents poured over the first third of that circle, the wider semi-circle in front of the door, and a few additional flourishes inspired by Gallifreyan and the Lesser Key of Solomon drawn within it). "That looks good."

  Gary tossed the other end of the string to me, making sure the length didn't fall across the line of salt between us, then walked towards my computer and the piles of junk around it. He picked up a coil of wire and returned to his own laptop, plugging one RJ-11 connector into a square plastic loop sticking out of the side of the computer.

  "I'm not touching that thing, by the way." He nodded at the DAC-amp-electrical hazard set between his circle and the door. I knelt in front of it, facing him and carefully unwinding the phone cable over the line.

  A pair of heavy purple rubber gloves lay pointedly next to the machine as I taped down the cable in front of and behind the salt circle. I then emptied the rest of the canister over it, closing the circle again with the wire buried under the salt and drawing a line about a foot or two long up its length. The result was a truncated computer power symbol. "You don't have to, just run the script when I tell you to."

  I plugged the other end of the phone cable into the DAC, which would spoof a modem handshake with Gary's computer to trick it into sending the raw data stream through the acoustic coupler, which would then send that signal into the amplifier, which would blast the bad math as raw electrical pulses into the door. I pulled on the gloves, trying to snap the bottom edge of one against my wrist theatrically. The rubber was too thick, and it instead flopped quietly.

  The acoustic coupler might have been a bit much, but I thought adding just a bit of natural interference to the stream would help make the end result even more satisfyingly crunchy and unpleasant.

  Gary looked between his screen and the amp, and started chuckling. I stepped inside the semicircle by the door and picked up two car battery clamps that connected to heavy wires running to the other end of the amp, sections already buried in salt to keep the diagram on the floor intact. I crossed the semicircle again, double-checked its integrity, then bent over the amp to connect a portable battery to the Arduino. The power light on its circuit board glowed a friendly green. Then I eyed the heavier cable that would power the amp, still unplugged from the power strip.

  "Almost ready on my end! How about you, Gary?" I watched my friend, who worryingly was still chuckling after all the cable-checking. "Gary?"

  His chuckles grew into a higher-pitched laughter he struggled to stifle. "Ready when you are! Ready for anything! Ready for nothing!"

  The laughter grew and he stared at me with completely black eyes, "It's getting louder. Void. Nothing. Something please fill it. Alex please. I'm scared. Void. Fill the void. Void fill it. Void please."

  Oh, shit. I don't have to say this was a bad sign. Still, his eyes were just black and not glowing, and he was at least somewhat still coherent. Fortunately, I had at least somewhat planned for this contingency. I had been rolling the implications of what was behind the door around my head for a few hours already, and I knew a few things about disruptive technology. Literally disruptive, not venture capitalist disruptive.

  Like I said, better to have and not need than need and not have. I rushed to the piles of junk and pulled out the handheld game system I was plinking away at in between nagging Gary about hearing any voices (which maybe was a good idea in the first place, huh, Gary?). It woke up from sleep mode and the icon on the screen showed me that a game was already running. "Stay with me, buddy!" I hopped over the circle, making sure I didn't break it, and knelt in front of him. Then I pressed the A button. The screen flashed white, then showed an array of dancing colors behind falling blocks.

  How do you protect a mind from visualizing arcane and lethal mathematics that can summon horrors? How do you purge a mind of strange patterns that can burn the soul? How do you replace the hackable source code of a foreign universe that can dig holes in someone's brain just by picturing it?

  The same way you get a song out of your head. With a stupider, catchier song.

  The Tetris effect is real. It saved my friend's life.

  Gary looked at the glowing rectangle in my hands, eyes black, cheeks glistening with tears. He focused on the dropping block, an L-piece slowly falling down a neon corridor where it would land in a space perfectly sized for it. The voids in his eyes shimmered, reflecting the bright fireworks that danced behind the game field (technically called the "well" in Tetris, and now you've learned something today).

  Several uncertain seconds passed before the L locked in place, filling in the squat pile of blocks at the bottom of the well. Three rows sparkled, then disappeared with a melodic chime. Gary blinked, and his eyelids lifted to show normal, conscious, human eyes. "Alex?"

  I'll be honest. I couldn't believe that worked.

  "Play your Tetris, Gary. But first, mind hitting that button? Keep an eye on the game."

  The mathemagician, properly half-hypnotized by a puzzle game, took his right hand off of the game system just long enough to tap on his touchpad. There was a soft series of tones as his laptop's modem dialed out through the acoustic coupler it was wired to. The coupler sang into the handset taped to it, and the Arduino connected to the handset responded by sending the right counter-tones to trick the modem into thinking it was connected to another computer over the phone. Well, technically it was, but not in a way it was remotely designed to do.

  Anyway, then the device started screaming. This was fine.

  If you're too young to know what dialup is, and it horrifies me that you might very well be, I'll explain the technology to you. Long before Wi-Fi and broadband in every home, we used to connect to the Internet through the phone. No, the landline phone. And there were computers that your computer would call through the phone line-

  You know what? Before you were born computers communicated by screaming numbers at each other. Moving on.

  With Gary safe and his laptop making horrible but nostalgic noises, I picked up the power cable connected to the amp and plugged it into the power strip. Then, with great care and fear, I flicked the switch on the power supply and winced.

  The fan of the power supply started to spin. The bare bulb of the vacuum tube wired into the amp glowed gently. Nothing burst into flames. This was good.

  I looked between Gary, the amp monstrosity, and the door as I pulled the heavy gloves off. With some satisfaction I nodded and stepped back. "If I'm right, that should do it."

  "Glad to hear it, buddy!" Gary's face was lit up with bursts of color, his eyes fixed on the irresistible hypnotism of Tetris.

  And so I watched the door and waited. For about a minute, nothing seemed to happen. I eyed the clamps on the door and the soft glow of the vacuum tube, and kept an ear on the shrieking modem connection. The door stayed there, unmoving, silent. Until...

  Knock.

  I jumped.

  It was the door. That door. A loud, definitive thump of notice from the other side of that plain wooden slab.

  Knock knock.

  Two more. I swallowed.

  "Someone's at the door." Gary mumbled offhandedly, still fixated on the game. His tone indicated he was in a typical state of pure distraction and not being called by the rapidly filling void glimpsed through forbidden charts, at least.

  Knock knock knock.

  It kept going. I thought about the possible horrors behind that door. I thought about distant, alien intelligences and realms of pure mathematics.

  Knock knock knock knock knock.

  I thought about power I had never been able to even touch finally being at my fingertips. I thought about patterns I recognized. I thought about the simple sublimating the endlessly complex. I thought about reality itself and the gears that meshed together to form it. I thought about the Platonic and the esoteric.

  I stared, mouth agape, as the knocking continued.

  Knock knock knock knock knock knock knock knock.

  I thought about a number, counting up, telling me that there was consciousness behind that door. I thought about Dr. Polchin's words.

  When there's a knocking on the door, turn the knob.

  I started to walk numbly towards the door.

  Knock knock knock knock knock knock knock knock knock knock knock knock knock.

  I thought about how it was an invitation, not a warning.

  When there's a knocking on the door, turn the knob.

  I reached towards the doorknob.

  Knock When knock there's knock a knock knocking knock on knock the knock door knock turn knock the knock knob knock knock turn knock the knock knob knock turn knock the knock knob knob turn knock the knock knob knock.

  Turn the knob.

  Turn the knob.

  Turn the knob.

  Wait.

  My palm made contact. With my forehead. With a hard slap. "I'm a fucking idiot!"

  I hopped back over the salt circle and bent over the amp. And I turned the potentiometer I soldered into the circuit clockwise as far as it would go.

  Potentiometer, also known as a variable resistor. Or, when used in an audio device to manage the total output of the circuit, a volume knob.

  Whatever was on the other side of the door was knocking to get a response. Because it couldn't hear what we were sending. Because the volume was turned down (which, when you aren't trying to blast a distant dimension with nonsense mathematics, is the best starting position when using any audio device so you don't destroy your ears because you forgot about how loud you set the output elsewhere in the signal chain).

  Was I being controlled by an alien presence and only broke free through sheer inspiration? Was I fully conscious and just having an almost catastrophic brainfart before I realized I had forgotten something obvious? Which one is less embarrassing?

  The laptop-to-Arduino screaming didn't get any louder and the vacuum tube didn't get any brighter, but the wires connecting the amp output to the doorknob and doorframe started to audibly crackle.

  I counted twenty-nine knocks on the door before they abruptly stopped. That's when I knew it worked, because there should have been thirty-four knocks. The most basic math progression, the Fibonacci sequence: one of the best ways next to counting only in prime numbers to indicate without words that you're an intelligence capable of some kind of conceptual abstraction.

  Also, the door abruptly stopped. That was another big hint.

  After the twenty-ninth knock, the door simply ceased to be. In its place was just, well, wall. Thankfully nonconductive wall, because the clamps connected to the running amp with two very live wires were fused to the wall in the exact positions they had been attached to the doorknob and frame.

  I ran over to the power strip and flicked it off. Then the power supply.

  The circles of salt on the floor were gone as well, or at least replaced with some nasty (but honestly very cool-looking) scorch marks. The Met and the Circle could handle the sanding and polishing on that job. And getting the clamps out of the wall.

  The amp, though...

  First, I disconnected the phone cable from Gary's laptop. The shrieking stopped. Then I unplugged the Arduino from the battery that was powering it. Then I put the gloves back on and grabbed the longest, most battered flathead screwdriver I could find in the piles of junk.

  Electricity isn't fundamentally magical, but it's just as dangerous. That's why you do everything you can to avoid short circuits, and to make sure any components that might hold a charge are discharged before handling. If you don't, it can get very noisy, very bright, and very hot, very fast.

  And that's without magic getting involved, which can result in, well... It was a long time ago, and I can still see and hear him screaming before I was blown out of a third-story window.

  The amp probably wasn't charged with any real magic, especially since the door disappearing probably meant the whole bleedthrough of metaphysical rules between universes was stemmed and there wasn't a danger of obliteration-by-computer anymore. But it still had a big honking capacitor in it, because that's how amps work. I covered my face with one hand and semi-blindly jabbed the screwdriver at the base of the capacitor with the other, touching the metal to both leads at once.

  There was a gunshot and a flash, then a wisp of smoke rose from the amp.

  Gary jumped with a squeal, finally broken free of his reverie by the sudden noise. "Ack!" He saw the blank wall. "Hey, we did it! Did we do it?"

  "You bet, bud." I grinned, shucking my gloves again. "But put down the Switch and tell me how the laptop makes you feel."

  He learned over his computer and tapped at the touchpad a few times, furrowing his brow. His eyes danced over the screen, flicking between different formulae and charts. "Let's see. This makes me feel... confusion, annoyance, some pity. Also a gnawing hunger inside me."

  Oh no. "You can still see a void?"

  Gary looked up at me, face dire. "Two deep, horrifying voids." Then he broke into a wide smile. "The void of your ability to understand anything past precalc, and the void in my belly because I'm actually pretty hungry. Let's grab some curry before you drive me home."

  ***

  I indeed drove Gary back to Staten Island, after we stopped for dinner (there's a great Japanese curry chain with a place near NYU). Then I stood over his shoulder as we both wiped and reinstalled his copy of Mathematica, and I made sure there wasn't anything else of the day's work. He didn't seem to mind, considering that without the foreign metaphysics of a distant universe trickling into the world it really was completely useless.

  We called Castle High Rock before I headed home and informed them of the extra door that had been expediently neutralized. The Circle would send a team the next day to investigate, and check the integrity of the actual shouldn't-be-there-but-at-least-we-know doors in the Cloisters.

  I also stopped by a computer store in Brooklyn on the way home (one of the few remaining big stores that still sells proper components and purpose-specific electronics) and picked up a fancy graphing calculator I would spend the next week poking and prodding at for Gary.

  The investigation and repair of the Merode room in the Cloisters meant the rest of my work would wait until they determined it was safe (and got the scorch marks out of the floor). Even if it wasn't the case, Bernard was taking the day off because of a migraine; even the lightest amount of futzing with time can have unpleasant side effects to those not used to it. I spent the next morning in my workshop, a cluttered and unassuming computer repair shop in the East Village.

  I was in a good, even playful mood, pointedly ignoring the small pile of electronics I needed to work on. I sat behind my desk, mug of coffee in hand, marker-vandalized paperback next to my keyboard, and waited for the inevitable.

  Three bells hung above the front door of the shop: one copper, one steel, one silver. Depending on the nature of the client, none or one of the bells would typically ring (I also had a piezoelectric chime to alert me of any non-Weird customers).

  All three half-rang as the door swung in, making an odd chirp. Grandpa Paradox has/have his/their own unique energy. And, lo and behold, a haggard and worried-looking Gustav Polchin lurched in.

  "Are you Alex Norton?" His eyes were wide and red.

  I stood up, taking a sip of my coffee before greeting him with my friendliest you-want-to-pay-me-for-goods-or-services smile. "Hey, Gramps! What brings you in today?"

  The temporally confused physicist walked to the counter. "My name is Dr. Gustav Polchin. I have to tell you-"

  "I know, Gramps. We've met."

  He shook his head. "No, I am not your Dr. Polchin. I'm from another world, parallel to yours but separate from any timeline."

  I nodded, "You're from an Earth distinct from ours through a variance in superstring vibration, adding an additional axis to the already n-plus-one axes you and my Grandpa Paradox typically traverse, yes?" Sip.

  He did a double-take, then sighed. "You've met me before."

  The shoe was so rarely on the other foot! "You'll come by yesterday. Nice for you to meet me now, though!" I offered him his own mug of coffee, freshly poured. He waved it off. "You can take a breath, you know."

  Dr. Polchin didn't seem reassured. "You being here, now, doesn't mean anything. I have to make sure the door never opens. And it opens today."

  I shrugged and put my empty mug down. "Hey, I get it. Which is why you're going to talk to me yesterday about this."

  He shook his head slowly. "I don't think you understand how dangerous this is. This is an impossible connection between worlds separated along lines even I shouldn’t be able to cross. Distant enough to have completely alien physics, but close enough in concept to transpose those physics over our own laws of nature if the connection is made."

  "Completely platonic universe with an entirely information-based metaphysical superstructure resulting in higher mathematics functioning effectively the same as magic does here, and that superstructure is populated with extradimensional horrors much, much more densely than any dimension adjacent to here. That's about the size of it, right?"

  Dr. Polchin eyed me incredulously. "That's more or less correct, yes."

  I eyed the paperback on my desk. I had the opportunity to do something very funny, at some risk to the space-time continuum. But I am a mature adult, so instead I turned and walked to the bookshelf behind me. I took the same paperback off of the shelf and returned to the counter Other Grandpa Paradox was standing in front of. "Thumb through this. It sounds like what you're talking about, and it'll help me understand it much better if you gave it to me. Oh, but first."

  I really don't like marking my books, but what was done was done, and I was about to do it. I might be a simple, three-dimensional-plus-one-additional-linear-vector creature, but I can at least respect the guidelines of relative causality. I took a Sharpie from the cup of pens on the counter and swiped it across the top of the paper back before handing it to Dr. Polchin.

  He froze. Or he moved very quickly. Or he read at a natural pace but returned to the moment he took the book after he was done. Whatever he did, he nodded at the paperback in his hand. "This is disturbingly accurate. Thank you, Mr. Norton."

  "Oh!" I snapped my fingers. "This is very important. You have to tell me that, when there's knocking on the door, turn the knob."

  This got a well-deserved blank stare. "When there's knocking on the door, turn the knob."

  "Yes."

  Dr. Polchin frowned. "You're sure you don't want to be more specific?"

  I grinned. "You sure you don't want to explain the intricacies of how my yesterday is your tomorrow and you’re afraid of a universe-consuming danger from that yesterday? Because my you has tried a few times and I still don't get it."

  He opened his mouth, then closed it. Then he tucked the paperback into his vest pocket. "When there's knocking on the door, turn the knob."

  "I'll know what that means. Eventually." I sat back in my chair and kicked my feet up on the desk. "Good luck, Grandpa!"

  And then he was gone, or was never there. And I put the safely time-looped novel back on my bookshelf. Then I spent the rest of the morning on Wikipedia trying and failing to get a grasp on superstring theory. And snickering about D-branes.

  Beware the door not there before. But if that door isn't there anymore, at least you can have some fun with it.