Chapter Text
Chapter 1: A Frequency Unknown
The last thing Hikigaya Hachiman remembered was rain.
Just the ordinary, inconvenient, Tuesday-afternoon kind that soaks through your school bag and makes the walk home from Chiba Station feel twice as long as it actually is.
The kind of rain that doesn't care about you. The most honest kind, really.
He remembered the sound of it against his umbrella. The rhythmic, indifferent percussion. He remembered thinking that at least the sound was consistent. Unlike people.
Then there was nothing.
Then there was this.
The first thing Hikigaya Hachiman noticed, upon regaining consciousness, was that the ceiling was wrong.
This was not a ceiling he had ever seen before. He had a reasonably comprehensive mental catalogue of ceilings — his bedroom ceiling, with its water stain shaped vaguely like Hokkaido; the Club Room ceiling, which he'd stared at often enough during those long, unstructured silences that it had practically become a personal acquaintance; the infirmary ceiling, which he'd also spent quality time with after the car incident, back when self-sacrifice was still something he thought he could afford. None of those ceilings were this ceiling.
This ceiling was enormous. Vaulted. Constructed from some material that seemed to be simultaneously metallic and organic, its surface threaded through with faintly luminescent veins — pale blue, the color of something that had once been warm and was now merely humming with memory. It curved overhead like the ribcage of something incomprehensibly large, and at its apex, several stories above him, a ring of crystalline structures pulsed with a slow, rhythmic light.
"Huh." he thought distantly. "That's not my ceiling."
His second observation was that he was lying on the floor. This was undignified in a way that felt specific and targeted.
His third observation was that something smelled like ozone and burning circuitry and, faintly, somewhere beneath those two things, pine — which made no sense whatsoever and which he filed under Things To Be Confused About Later.
He sat up. The motion cost him more than it should have. His body felt like it had been turned inside-out and then put back together by someone working from a diagram that was missing several key pages. His vision swam, resolved, swam again, and finally settled into something approximating functional.
The room — if "room" was even the right word for a space this large — was a chamber. A research chamber, by the look of it, though unlike any research facility he'd seen in photographs or movies or the occasional science documentary Komachi had insisted on watching on weekend mornings. The architecture was aggressively advanced in a way that felt almost theatrical, like someone had tried to design what "the future" looked like and had overshot into a register somewhere beyond it. Banks of instrumentation lined the curved walls, their displays alive with cascading data in scripts he couldn't read. Concentric rings of some kind of engineered material were inlaid in the floor around a central raised platform — the platform he had apparently just been lying on top of.
There were scorch marks on the floor. They radiated outward from the platform in patterns that suggested something had discharged with considerable force, very recently, in all directions.
He was, as best he could tell, at the epicenter.
"Okay." he thought, in the particular calm register he reserved for situations where panic would be both appropriate and useless. "So something exploded, and I'm in the middle of it. That's fine. That's completely fine. This is fine."
He stood. His legs held. Another small, unearned victory.
The chamber extended around him in every direction, and beyond a row of blast-shielded glass — several panes of which were cracked — he could make out an observation gallery. It was currently empty of people, though the scattered equipment and the still-open research terminals suggested that people had been there very recently. Someone had knocked over a chair in what appeared to have been significant haste.
He looked down at himself. His school uniform was intact, though rumpled and slightly singed at the edges of his sleeves. His bag was gone. He was still wearing his shoes, which struck him as important in a way he couldn't quite articulate.
He looked at his hands. Both present. Ten fingers, correctly distributed. His left hand had a faint redness to it — like the afterimage of something very bright, or something very hot.
He looked at his right hand.
He stared at his right hand for a long moment.
Then he turned it over and looked at the back of it. Then back to the palm. Then the back again, more slowly.
"...that's new."
The underground sky of Lahai-Roi was not a sky in any sense that Hikigaya had a word for.
He found it approximately twelve minutes later, after navigating out of the research chamber through a door that opened with an enthusiastic hiss when he approached it — which was either a welcoming sign or a deeply impersonal automated function, and he suspected the latter — and down a corridor that opened onto an external observation deck. The corridor itself had been remarkable enough: clean white surfaces interrupted by exposed structural elements that pulsed with the same low bioluminescence as the chamber ceiling, holographic signage in a script he couldn't read, the distant hum of machinery that seemed to come from the building itself rather than any specific point within it.
A building that was alive with its own circulation. He walked past laboratories behind transparent walls, past clusters of equipment that defied his ability to categorize their function, past an automated cart that trundled along a track set into the floor and which regarded him with a camera eye as it passed.
No people. Which was fine by him, generally, but which was also slightly worrying given the circumstances.
The observation deck opened onto the answer to a question he hadn't known he was asking.
There was no sky above him. There couldn't be — the architecture made clear he was underground, and had been since the moment he arrived. But what was above him, stretching across the ceiling of the vast subterranean cavern that housed this entire city, this entire civilization carved into the bones of the earth — was something that performed the function of sky so capably that the distinction felt almost pedantic.
Bioluminescent formations covered the cavern ceiling in constellations — clusters of organic light-producing structures, shaped somewhere between coral and crystal, throwing their cool illumination across a hidden world. The city below was enormous. Towers rose from the cavern floor in clean arcs, their upper floors lit from within, connected by elevated walkways and transit lines and what appeared to be some kind of guided transport system running along visible rails. Beyond the towers, the cavern extended further than he could see — fading into a luminous distance, warm and impossible and vast.
He stood at the railing of the observation deck for a long time. The wind — there was wind, somehow, even here — moved against his face with a humidity that reminded him of late summer evenings, which was the wrong memory to be having right now but which arrived anyway, the way inconvenient things always did.
He thought about his walk home from the station. About the rain. About the very ordinary, unremarkable afternoon that had apparently ended with him being relocated to somewhere that was not, in any identifiable way, Chiba.
He thought about Komachi. About whether she'd come home to an empty apartment, and how long it would take her to start worrying, and how she'd probably downplay the worry with jokes until she couldn't anymore.
He thought about — and then he stopped thinking about, because thinking about the club room right now, about its particular quality of afternoon light, about the two people who occupied it alongside him, was not something he could afford to do and maintain the studied blankness that was currently his primary coping strategy.
"Right." he thought instead, briskly, internally, in the voice he reserved for making decisions in the absence of better options. "Unknown location. Advanced technology. Zero contextual information. No contact with anyone. No way home currently visible. This is all extremely consistent with scenarios that are outside my range of experience."
He paused.
"...I've been isekai'd, haven't I."
He leaned his arms on the railing and looked out at the impossible, subterranean sky for another long moment.
"I don't even like those stories."
It started as warmth.
He noticed it when he turned away from the railing — a warmth in his right arm that hadn't been there before. Or rather, it had been there since he woke up, in the background, low and steady, easy to ignore in the presence of everything else demanding his attention. But now, in the relative quiet of his own thoughts, he noticed it more clearly.
He pushed up his right sleeve.
The warmth had a shape.
From just above his wrist, tracing upward along the inside of his forearm, something was forming. Not a bruise, not a burn — it was too precise for either. A pattern. He watched it in the ambient glow of the cavern light, and what he saw made no sense and was happening anyway, which was very on-brand for the last half-hour of his life.
Lines, thin as ink, were tracing themselves across his skin in a color that wasn't quite silver and wasn't quite blue — something that seemed to borrow from both without fully committing to either. The pattern moved with intention. Deliberate. It wasn't random scarring or circulatory trauma. It was *forming in the way that a drawing forms, stroke by stroke, as if some unseen hand was working upward from his wrist toward his elbow.
He stared at it with the calm of a person who has already processed so much in such a short window of time that their alarm response has temporarily gone out of service.
"Lines" he thought, carefully. "On my arm. Appearing by themselves. That's—"
They spread past his elbow. He rolled his sleeve up further. Past the elbow, to his bicep — the pattern widened there, the lines branching into something more complex, something almost geometric except for the places where it curved in ways that geometric patterns didn't, following the contour of muscle like it knew where it was going. Like it had always been here and was only now becoming visible.
The warmth deepened. Not painful. Not yet. But present in a way that suggested it was still deciding.
He stood very still in the wind of the underground world and watched the mark travel upward. Past his shoulder. He couldn't see it past that point — it moved beneath the collar of his shirt and he was not in a position to investigate further without causing a scene for an audience that didn't currently exist — but he could feel it continuing. Across his collarbone. Down his shoulder blade, maybe. Up toward the base of his neck.
The light in the lines pulsed, once. Twice. Like a heartbeat finding its rhythm.
Then it pulsed a third time, harder, and the warmth became something else entirely. Not pain. Not quite. A pressure, from inside, building fast and then faster, like a frequency being turned up to a volume the instrument wasn't designed for.
"Oh." he thought, with great simplicity.
His knees decided, independently, that they were done for the evening.
The observation deck rushed up to meet him, and then it was dark, and the underground sky of a world that was not his world went away, and Hikigaya Hachiman was unconscious again before he'd had the chance to come up with a suitably cynical observation about the experience.
In the research chamber behind him, several kilometers of instrumentation came suddenly, catastrophically, and inexplicably alive — spiking into readings that had no established precedent in the database — and fourteen researchers, who had only just cautiously returned after the initial explosion, scattered from their stations for the second time in the same hour.
One of them, a second-year with quick hands and quick eyes, caught the data before the displays crashed.
She stared at what it said for a long moment. Then she looked toward the observation deck with an expression that was caught precisely between scientific excitement and genuine alarm.
"Someone get Professor Mornye," she said. "Right now."
The mark on Hikigaya Hachiman's arm continued its slow, silver pulse, steady and patient as a star that had been burning long before anyone looked up to notice it.