The staircase creaked.

  Not randomly — each step had its own tone, its own particular complaint, like a poorly tuned instrument that nonetheless followed a score. Crowe climbed slowly, one hand grazing the brick wall, counting the sounds without meaning to.

  The first-floor corridor had two doors. The one on the right was Voss's flat. The one on the left belonged to the neighbour — the one who had heard the glass and waited until dawn.

  Crowe stopped in front of the left door first.

  He knocked three times. Paused. Knocked once more.

  Footsteps from inside. Slow, cautious — the kind of steps from someone already awake but pretending not to be. The door opened as far as the chain would allow — about three inches — and an eye appeared in the gap. An older man, white hair dishevelled, flannel nightgown. The smell from inside was cold tobacco and tea forgotten on the stove.

  — Already spoke to the inspector — the man said.

  — I know — said Crowe. — I have just one question.

  The eye examined him for a moment.

  — One — said the man.

  — When you heard the glass break — said Crowe — what did you do in the first thirty seconds afterward?

  Silence. Not the silence of someone who doesn't know the answer — the silence of someone who hadn't expected that particular question.

  — I stayed still — the man said, at last.

  — Still where?

  — In bed.

  — Eyes open or closed?

  The door chain clinked faintly, as though the hand on the other side had tightened around the frame.

  — Closed — the man said. And then, quieter: — I couldn't open them.

  Crowe was quiet for two seconds.

  — Thank you — he said.

  He turned before the man could ask anything further, and heard the door close with the particular care of someone who doesn't want the other side to know how relieved they are that the conversation is over.

  Voss's door was unlocked.

  That was the second unlocked door of the night, which was a data point. Not necessarily an important one yet — but Crowe didn't discard data before having a framework to accommodate it.

  The flat was small and honest.

  A room that was also a study, a narrow kitchen at the back, a half-open door to what must have been the bedroom. The walls of the main room had the same relationship with space as the shop below — covered, occupied, nothing wasted. But where the shop had clocks, the flat had paper.

  Sheets everywhere. Pinned to the walls, stacked on the table, spread open in books that served as weights for other sheets. Calculations, diagrams, annotations in small handwriting running along the margins and continuing on the reverse without warning. There were at least four different languages Crowe identified on a first sweep, and a fifth he didn't recognise.

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  The lamp on the table was out, but warm. It had been used recently.

  Crowe stood in the centre of the room for a moment without touching anything, simply looking. That was what he did in unfamiliar spaces — let the room say what it wanted before he started asking questions.

  What the room said was this: Aldric Voss had been a methodical man who had recently lost his method. The older stacks of paper had an internal order — numbered, dated, organised by subject. The more recent ones were chaos. Sheets mixed together, calculations abandoned mid-way, annotations that began in one direction and abruptly changed to another.

  Something had shifted in Voss's work over the past few weeks.

  Crowe moved toward the most covered wall — the one directly above the desk. There the sheets were newer and the density greater, as though Voss had progressively migrated toward that specific point. At the centre was a diagram larger than the others, drawn directly onto the wall in graphite.

  Concentric circles. Lines radiating from the centre. Numbers at specific positions along the lines, following a pattern Crowe studied for twenty seconds without being able to name — it was not conventional trigonometry, not musical notation, not any system of measurement he recognised.

  But there was something familiar in the overall structure.

  He tilted his head slightly.

  It was a mechanism. Not drawn as a technical blueprint — drawn as if seen from the inside, as if someone had tried to represent the workings of something that had no conventional physical form.

  At the centre of the diagram, where the main axle of a mechanism would ordinarily sit, Voss had written a single word.

  Just one.

  Interval.

  Crowe looked at it for a longer moment than he had looked at anything else that night.

  Then he took the note from his waistcoat pocket and opened it beside the diagram.

  He saw the interval.

  It wasn't a warning. It was a confirmation. Voss had not written it in fear — he had written it with the precision of someone recording a conclusion. Like a scientist noting an experiment's result in a notebook before leaving.

  He saw. And then he was gone.

  Crowe folded the note carefully and put it back.

  He went to the flat's window. It faced the street — the same stretch of pavement where he had stood smoking minutes before. The fog had thickened, and the lamps below were now only yellow smears floating in the white.

  He looked down for a moment.

  Then he noticed.

  From the flat's window, in a straight line directly below, was the shop window. The broken window. And if Voss had been here, in this flat, looking down while working on the diagram on the wall —

  Crowe stepped back from the window.

  There was an overturned chair in the corner of the room, which he had noted on entering and filed as a minor detail. Now he went to it. It was upturned with the seat facing upward, near the side wall — not near any table, not near any shelf. Midway between the door and the window, with no apparent purpose.

  Except that the seat was at the wrong height for something that had simply fallen. If a chair topples forward or backward, the seat hits the floor. This one was facing up, with the legs pointing sideways, as though it had been placed that way deliberately, or as though it had rotated in the air before landing.

  Crowe crouched down.

  On the floor beneath the chair, nearly invisible against the dark wood, there was a mark. A small circle, roughly the diameter of a large coin, burned into the wood with a regularity that ruled out accident. Inside the circle, a line. A single straight line, crossing through the centre.

  The same symbol that appeared repeated in the margins of Voss's most recent sheets.

  Crowe stayed crouched, looking at it for a long moment.

  Headaches were not uncommon for him — they sometimes came after long cases, pressure behind the eyes, passed quickly. But the one that began at that moment was different. It arrived suddenly, on the left side, a single sharp stab and then a dull weight that stayed.

  He stood slowly.

  The flat was silent. Outside, Londrivar groaned and breathed steam, as indifferent as ever to what happened within its walls. Somewhere on a floor below, Maren had probably given up finding anything else useful and was waiting outside with the expression of someone who would rather be in bed.

  Crowe took one last look at the diagram on the wall. At the word in the centre.

  Interval.

  He didn't know what it was yet. He didn't have enough data, enough framework, enough of anything beyond fragments that touched at the edges without forming a complete image.

  But he knew one thing.

  Aldric Voss had spent weeks — perhaps months — trying to understand something. He had reached a conclusion. He had written that conclusion on a note and hidden it where an attentive person would find it. And then he had vanished from a sealed room without leaving a trace.

  Which meant that understanding had not been enough to protect him.

  Crowe extinguished the lamp and left.

  The headache went with him.