Harlan Vey's flat was on Linden Street.

  Three blocks exactly from Acacia Street — Crowe had counted without meaning to, the kind of automatic count that happened on short journeys when the mind was occupied with something else. Both streets were in the Upper Quarters but of different generations — Acacia Street had the width and building spacing of the older construction, when there had been enough money to build with generosity. Linden Street was more recent, more compact, with townhouses closer together in a way that in the Upper Quarters was discreet and in any other neighbourhood would have been ordinary.

  The building at number four Linden Street was yellowed brick with wrought-iron details on the windows — ornament that had been elegant when installed and that time had converted into habit. Four floors. Vey's flat was on the third.

  Vey's wife opened the door before Crowe knocked.

  She had been waiting — it was visible in the way she opened it, with the speed of someone who had been near the door for some time. Thirty years old, black hair pinned up with the particular carelessness of someone who had pinned it without a mirror, eyes that had passed through the full sequence of that morning's emotions and arrived in a region of taut determination.

  — You're the investigator — she said.

  — I am — said Crowe.

  — The guard said my husband is fine — she said. In the voice of someone verifying information they had received but had not entirely managed to believe.

  — He's fine — said Crowe. — Confused, but no physical damage.

  She stood in the doorway for a second. Then stepped back and let him in.

  The flat was smaller than the one on Acacia Street but had the quality of a place inhabited with care — not with money, with attention. The objects had positions that were choices and not chance. There were books on a shelf that showed real use, with spines worn in the uneven way that indicated reading rather than decoration.

  In the sitting room, an armchair by the window with an almanac on the armrest.

  Crowe went to it. The wife stood in the space between the sitting room and the corridor, watching with the attention of someone who had decided to cooperate but had not yet decided to trust.

  The almanac was old — leather cover with the title in worn gold letters: Almanac of Applied Mechanics, Vol. III. Crowe opened it. The marked page was about gear systems — diagrams of mechanisms, torque calculations, the kind of content a man who worked with mechanics read for pleasure or out of habit.

  There was a fold in the page. Recent — the paper was still slightly resistant to returning to a flat position, which indicated it had been folded that night or the night before.

  Crowe looked at the diagram on the marked page.

  It was a clock mechanism. Not an ordinary clock — a precision mechanism with multiple axles, the kind used in scientific measuring instruments. The diagram showed the mechanism from the front, with all the gears visible and numbered.

  At the centre of the diagram, where the main axle would sit, there was an empty space.

  The same empty space that had been at the centre of Voss's diagram — the one drawn on the flat wall, with the word interval written in it.

  Crowe looked at the diagram for a moment.

  Mechanics almanacs. Diagrams with an empty centre. Reed, Vey — and Voss, who had spent weeks studying exactly this kind of mechanism before disappearing.

  It was not a coincidence of reading. It was resonance. People with liminal sensitivity were drawn to visual representations of the mechanism that described the interval without knowing they were being drawn.

  — Does your husband read this almanac often — said Crowe, turning to the wife.

  — Since his father left it to him — she said. — For years. — A pause. — Always that volume. Never the other two.

  — Always open to this page — said Crowe.

  She blinked.

  — How do you know — she said.

  — The fold — said Crowe, showing the corner of the page. — Several overlapping folds. He always marks here when he stops reading.

  The wife looked at the almanac with an expression navigating between recognition and something she didn't know how to name.

  — I'd never noticed — she said. In the low voice of someone realising they had been looking at something for years without seeing it.

  Crowe closed the almanac carefully. Put it back on the armrest exactly where it had been.

  — May I see the rest of the flat — he said.

  The wife accompanied him in silence.

  The flat's internal corridor was short — sitting room, bedroom, a smaller second room that served as a study, kitchen at the back. The layout was simple, functional, with nothing that stood out on a first sweep.

  Crowe moved slowly, lamp held low — he had asked permission to close the curtains before starting, which had left the wife with the expression of someone trying to understand the request and deciding it was simpler just to agree. With less natural light, the lamp revealed details that daylight levelled out.

  In the bedroom: nothing. Bed made with the precision of someone who had made it to have something to do while waiting. Wardrobe closed. A bedside table with a glass of water and a pocket watch.

  Crowe moved closer to the watch.

  It had stopped. Not fast, not slow — stopped. The hands showed twelve twenty-two.

  Reed. Twelve twenty.

  The difference of two minutes could be chance. Could be the margin of imprecision of two different watches. Could be exactly what it appeared.

  The author's narrative has been misappropriated; report any instances of this story on Amazon.

  Or it could be that the interval didn't stop clocks at the same moment — it stopped them at slightly different times depending on the point of entry. Like two clocks in the same time zone showing slightly different times because they had been set against different references.

  Crowe copied the time in the notebook. Put it away.

  He continued.

  The study was where Vey worked with things of personal interest — there was a desk covered in papers organised with the particular system of someone who knew where everything was but which would be difficult for another person to decipher. Technical books, manuals, two stacked notebooks.

  Crowe opened the top one.

  They were work notes — calculations, measurements, the kind of record a technician made during service. He leafed backward.

  On the twelfth page there was something different.

  It was not a work note. It was a drawing — made with the same pen as the annotations, in the same handwriting, but with a different quality of intention. Like something drawn from memory, without reference, in a moment of a different state from usual.

  It was the symbol. The circle with the line.

  Not identical to what Crowe had seen in the other occurrences — there were small variations, the circle slightly irregular, the line not perfectly centred. But recognisable. Unmistakably the same symbol.

  Below the drawing, in smaller and more hurried handwriting than the rest of the annotations:

  It was there again. The same dream. The corridor. The door at the end. Something on the other side that isn't a threat but isn't anything I have a word for. I woke with this drawing done without knowing I was doing it.

  Crowe looked at the notebook for a moment.

  The date in the corner of the page was three weeks ago. Before this morning's incident. Before any contact with Crowe's investigation, with the Guard, with anything that could have introduced the symbol into Vey's life in an explicable way.

  He had been dreaming of the corridor. Had been drawing the symbol without conscious intention.

  Vey's liminal sensitivity had not begun this morning. It had been growing for weeks.

  Perhaps longer.

  — Does your husband have nightmares often — said Crowe, without turning.

  The wife was in the study doorway. She was quiet for a second.

  — They're not nightmares — she said. Carefully. — He doesn't wake with a start. He wakes quietly, sometimes with things drawn in the notebook that he says he doesn't remember drawing. — A pause. — It started about two months ago. I asked him to see a doctor. He said he was fine.

  — Two months — said Crowe.

  — More or less.

  Crowe closed the notebook. Returned it to the pile in the same position.

  Two months of dreams about the corridor. Three weeks of the symbol appearing in notebooks without conscious intention. And this morning — the partial crossing. Not complete like Voss's, but enough to displace him three blocks through space.

  Vey was being prepared. Not by the Architect directly — by the interval itself, by the growing proximity to the active point that existed at a specific address in that part of the Upper Quarters that Crowe had not yet identified but that was becoming easier to triangulate with each new data point.

  He left the study. Went to the corridor.

  And it was in the corridor that he found what he had come to find.

  The threshold of the bedroom door.

  It was stone — the same material as the corridor, the same colour, the same wear. Except at one specific point, thirty centimetres from the right wall, where there was a mark that normal wear did not produce.

  Crowe crouched down with the lamp.

  It was the symbol. Not engraved — burned. The same quality of mark he had seen on the floor of Voss's flat, in the corner where the chair had fallen. But this time there was no chair. There was only the threshold and the mark and the residual heat that the lamp revealed as a subtle variation in the way the surface reflected light.

  Recent. Very recent.

  — Was this here before — said Crowe, to the wife who had followed him into the corridor.

  She looked at the threshold. At the mark. She was quiet for a moment with the expression of someone processing the distance between what they were seeing and what they had expected to see.

  — No — she said. With the certainty of someone who cleaned that corridor regularly. — It wasn't.

  — From this morning — said Crowe.

  It was not a question. The wife didn't answer as though it were.

  Crowe crouched looking at the mark for a moment.

  There were two things the symbol on the threshold meant and both of them were unsettling in different ways.

  The first: Vey's liminal entry point was here. This specific threshold, in this corridor, was where the interval was thinnest in this flat. It was the point of least resistance between ordinary space and the Threshold — the gap between the teeth, in Voss's language.

  The second: the mark was from this morning. Which meant it had been made during or immediately after Vey's crossing. Not by Vey — he had crossed and woken three blocks away, he had not come back. It had been made by something else.

  The Architect marked the entry points.

  Not for investigators — for themselves. As cartography. As maintenance of a map of points where the Threshold was still thin enough to function.

  Crowe copied the position and dimensions of the mark into his notebook. He made the sketch with his usual precision.

  He put it away. He stood.

  — I'll need you and your husband to remain accessible for the next few days — he said, addressing the wife. — If your husband has any similar episode — a dream, involuntary drawing, anything out of the ordinary — I want to know immediately.

  — Is he in danger — she said. Directly. Without the qualification most people placed around questions whose answer they feared.

  Crowe looked at her for a moment.

  It was the kind of question that deserved an honest answer, which made it the hardest kind of question to answer.

  — I don't know yet — he said. Which was the only true thing he had.

  The wife looked at him. Then nodded once with the firmness of someone who had received an unsatisfying answer and had decided it was at least honest.

  Crowe left.

  On the pavement of Linden Street he stood still for a moment with the notebook in hand and the new data settling.

  The almanac always open to the same page. The watch stopped at twelve twenty-two. The symbol drawn without conscious intention three weeks ago. The burned mark on the threshold from that morning.

  And the corridor. The door at the end. The presence that was not a threat but had no name.

  There was something that had been forming since the interview with Vey and which now, on the pavement, had risen to the surface with the particular clarity of a conclusion that had been waiting for the right moment to appear.

  Reed had said: mechanics almanac. Twelve twenty. Vey had said: mechanics almanac. Twelve twenty-two.

  Two men with liminal sensitivity, at two different active points in the city, reading the same kind of material at the moment the interval had manifested.

  It was not a coincidence of reading.

  It was a signal.

  The interval manifested with greater force in people who were, at that moment, directing their attention toward mechanical representations of the phenomenon itself. As though there were a tuning frequency — and mechanics almanacs with gear diagrams were, inadvertently, the closest available means of tuning to that frequency without knowing that was what one was doing.

  Voss had discovered this deliberately. He had built an entire diagram on the window as a way of amplifying the tuning. He had managed to see the interval with a clarity that the other two had not achieved.

  And he had disappeared completely.

  Reed and Vey had reached the liminal state by accident, without amplification, without preparation. They had come back — or almost come back, in Vey's case — because they had not gone far enough for the crossing to be complete.

  The degree of tuning determined the depth of the crossing.

  Crowe stood on the pavement looking at the empty Upper Quarters street.

  He had in his coat pocket, since Case 001, the sketch of Voss's diagram. He had looked at it dozens of times during the investigation.

  He also had the circular mark on the pavement in front of Voss's shop. The instrument that had measured the active point before the disappearance.

  And he had, in his waistcoat pocket, Ashcroft's note saying there was something that could not be sent by note.

  He took out the notebook. He added to the sketch of Vey's flat a line connecting the threshold position with the direction of the Upper Quarters toward Acacia Street.

  Then he added a second line, estimated, in the direction of the nearest active point he had triangulated from the available data.

  The two lines converged on an address.

  It was not number seven Acacia Street, which was the wrong flat where Vey had woken.

  It was number twelve. Two buildings further along. An address that had appeared in no report, no interview, no document he had consulted.

  Crowe looked at the address in the notebook for a moment.

  Then he put the notebook in his pocket and started walking.