The corridor the system offered after the junction was wider than the one before it.

  That was the first thing Griff didn't trust.

  The air came next — filtered clean, none of the discharged residue that had been sitting in the previous passage. Temperature steady. The kind of steady that wasn't climate. The kind that was maintained by something that had decided the crew deserved a comfortable corridor and was waiting to see what they did with it.

  Jake moved at point with his head at working height, ears in the tracking position. His tail held the working arc. Whatever had happened at the junction was behind them. The thread beneath Don's bandage was steady now. Constant. Valley had said permanent and permanent was what it looked like — a thin silver line visible through the fabric in anything brighter than corridor lighting, not pulsing, not responding. Just there. Don kept his arm at his side and didn't look at it and Griff watched him not look at it and said nothing.

  They'd made it forty metres before Jake stopped.

  His whole body changed at once — weight dropping back, head lifting, ears pivoting with the particular sharpness he used when what he was reading wasn't in front of him but above. His nose worked the air. Not forward. Up.

  Griff's fist came up.

  The formation locked.

  Silence pressed in around the sound of their own breathing.

  Above them, three metres up on the structural rib that ran the length of the corridor — something. Dog-sized. Eight legs, each one articulated at two points, the movement low and deliberate along the steel. It wasn't running. It wasn't dropping. It was keeping pace with where they'd been two seconds ago, and when the formation stopped, it stopped, and it settled its weight onto the rib with the patience of something that had all the time available and knew it.

  It just watched.

  "Valley," Griff said.

  "Single unit." Valley had the rig up, sweeping. "Heavier chassis than the Lads. Fewer sensors. The signal pattern is — observational. It's not reading formation geometry. It's reading individuals."

  "What does that mean?" Kyle said.

  "It means it's cataloguing us separately. Not the group. Each person."

  The silence that followed had a specific texture. Griff felt the formation absorb it without speaking — the particular discomfort of understanding that something above them, outside reach, was assembling a file on each of them individually and had probably been doing it for longer than they'd known it was there.

  Jake hadn't moved. His gaze was fixed on the thing on the rib, body held with the careful stillness he used when the noise he was considering making would change the situation in ways he hadn't finished calculating.

  Griff looked at the gap between ceiling and floor. The rib was three metres up. Jake had the launch capacity if there was a surface — but there was nothing below that rib. Just air and the thing moving freely at a height nothing on the ground could reach.

  The system had done its homework on the Lads.

  "Options," Griff said.

  "Mosin," Elin said.

  "Acoustics in here would cost us more than it costs the drone."

  "Rig," Valley said, already adjusting the dial. "At this angle I'd be pushing interference into the rib infrastructure. I don't know what's running through those ribs."

  "So we leave it," Scarab said.

  "We move," Griff said. "Formation tight. Watch the ceiling at junctions. Don't stop unless I say."

  "It already knows how we walk," Valley said, not as a complaint. Just placing it.

  "Then we give it something it can't use." Griff looked up at the thing on the rib. It hadn't moved. "Keep moving. Irregular pace. Don't settle into rhythm."

  They moved.

  The thing on the rib moved with them.

  Two seconds behind. Exact. Patient. Every time the formation shifted pace it absorbed the change and recalibrated within three steps, finding the interval again like a metronome correcting for a stumble. It didn't rush. It didn't react to the pace changes as problems — it logged them, updated its read, and kept following. Griff watched Archie try a false stop — the formation halting, the thing on the rib halting two seconds later, Archie stepping forward again. The thing followed. It had the interval by the third repetition. After that it didn't need to wait for the formation. It was predicting where they'd be.

  "It's learning faster than the Lads did," Valley said, quiet enough that it wasn't for everyone.

  "I know," Griff said.

  "The Lads needed three passes to read formation geometry. This one — two pace changes."

  "I know."

  The corridor ended at a junction. Left or right, both passages continuing at the same height, same width, same administrative lighting. The thing on the rib stopped at the ceiling corner above them and waited.

  Valley swept both passages. "Left: open signal. Right: blocked sixty metres in. Original construction collapse — a section of the old ceiling has come down across the passage."

  "The system didn't close it," Elin said.

  "Building did. Long time ago."

  Griff looked up at the thing on the ceiling. It hadn't shifted. It had positioned itself directly above the junction — the single point from which it could watch both passages, both decisions, every face. Cataloguing. He could feel it reading him and he couldn't do anything about that and the particular frustration of that was something he kept exactly where it was and didn't show.

  "Right," he said.

  "The blocked passage?" Scarab said.

  "The system doesn't know what's in there." He looked at the ceiling. "And if the passage is blocked by old construction, there's maintenance access around it that pre-dates the system's modification layer."

  The obstruction was exactly what Valley had described — original ceiling material across the corridor floor, old composite over older substrate, nothing the Hollow had placed. Behind it the passage continued, visible through a gap too low to walk. Above the obstruction, a maintenance hatch in the wall. Old steel. Manual latch. The system's modification layer stopped clean at its edges.

  Valley swept the hatch. "No signal inside the duct. The system hasn't mapped it."

  Griff looked up.

  The thing from the rib had followed them to the obstruction. It sat at the wall-ceiling junction, legs braced against both surfaces, watching the hatch. Its sensor cluster — the instruments where a face would have been — was oriented at the hatch with the particular attention of something that had just encountered a variable it hadn't accounted for.

  Eight legs. Built for the ribs. The chassis too wide for a twenty-five centimetre access hatch.

  "Duct," Mad said. He was looking at the span of the chassis, the hatch, back to the chassis. "It can't follow."

  The thing on the ceiling didn't move.

  It knew.

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  Getting the hatch open took Mad four minutes and a length of composite wedged into a latch that had seized with decades of disuse. When it gave, it gave all at once — the latch releasing with a crack that echoed and the thing on the ceiling adjusted its grip fractionally, logged it, held its position.

  "Jake," Griff said.

  Jake assessed the opening — forty centimetres by fifty, darkness smelling of old metal and trapped air — and went in with the economy of a dog who had been in tighter spaces than this and treated them as information rather than obstacles. Low and efficient. The tip of his tail disappeared last.

  The crew fed through in order.

  Mad first, pack ahead of him, immediately encountering the specific problem of his own shoulder width in a duct that had been designed for access, not for Mad. He navigated it on his elbows with the expression of a man who had profound personal opinions about whoever had specified the dimensions and was keeping those opinions contained because the duct was real and the opinions were not useful. He kept moving. Didn't speak. Breathing was currently allocated elsewhere.

  Archie second. His ribs found the floor on the second metre and paid for it — a sharp intake of breath that he shaped into a controlled exhale before it became anything the formation needed to respond to. The elbow-crawl had a specific cost and he'd been paying it since the Lads and he'd developed the technique for paying it without announcement.

  Valley. Elin. Scarab. Don — arm held clear, bandage catching the light from Kyle's lamp ahead. Kyle last, the hatch closing behind him.

  Thirty metres of duct. Then the exit.

  Mad's boot against it, the latch giving with less resistance than the entry, air coming through first — same temperature but more volume behind it, the corridor on the far side.

  They came out one by one.

  Griff counted.

  Everyone.

  Jake was already facing the ceiling above them.

  The thing wasn't there.

  Griff looked both ways along the corridor. Looked at the rib infrastructure — same design, same steel framework, the same surfaces the chassis had navigated at a confidence that suggested it knew this level better than they did.

  It had repositioned.

  Of course it had repositioned.

  It had seen the hatch. It had calculated the exit point. It had thirty metres of ceiling infrastructure between entry and exit and it had spent the time they were in the duct finding another route.

  Jake's ears pivoted — twenty metres down the corridor, a junction in the ceiling where two rib lines met. A maintenance ledge, original construction, a flat section of steel above the reacquisition point. The thing was already moving along the rib toward it, legs working the steel, pace unhurried. It had calculated the timing. It was arriving at the speed it had determined was sufficient.

  It had been right about the timing every time so far.

  Griff opened his mouth.

  The thing dropped.

  It came off the rib without warning — no shift in posture, no telegraphing — and it hit Archie at shoulder height with the flat of its chassis, the impact drove him sideways into the corridor wall with a sound that was ribs and composite and Archie's sharp cry cut short as the air went out of him.

  Jake launched.

  The thing was already moving — rerouting, one pair of legs finding the wall surface and driving it laterally across the corridor, the chassis keeping between Jake and Archie while two more legs reached for Archie's pack. Not attacking. Acquiring. It had read the formation. It had read Archie's existing rib damage every time he'd paid for it in the duct, in the elbow-crawl, in the way his hand found his left side when he thought no one was counting. It had built a file on each of them individually and it had chosen correctly.

  "GET IT OFF—" Scarab had the pry bar moving already.

  Mad hit it from the side with his shoulder — not the elegant leverage geometry he'd used against the Lads but brute contact, weight into the chassis wall, and the thing shifted but didn't release Archie's pack, legs finding the floor now, four legs on the ground and four managing Mad and the pack and rerouting simultaneously, more legs than they had counters for.

  Jake got underneath it.

  What Jake did wasn't an attack. It was geometry — his body driving upward into the gap between two of the chassis legs, his weight pushing the stable base wider than the thing's lateral tolerance, and the grip on Archie's pack faltered for a half-second as it redistributed.

  Scarab's pry bar found the sensor cluster.

  Not a clean hit. Glancing. But the cluster went dark on one side and the thing's orientation shifted — two legs losing their read on Archie, the chassis turning toward the new threat input.

  "PULL ARCHIE," Griff said.

  Elin already had him. One hand in his collar, dragging him backward along the wall, his boots finding purchase, a sound coming from his ribs that he was trying very hard to keep inside his chest.

  The thing recalculated. Archie was no longer available. It reoriented toward the next variable. Valley. The rig. Its sensor cluster — half-dark now — sweeping toward the phase equipment.

  Valley stepped back.

  "I can't get a clean pulse with the crew in the envelope—"

  "Don't," Griff said. "Mad."

  Mad was already looking at it with the expression he used for structural problems. At the chassis width. At the rib it had used for the drop. At the maintenance ledge twenty metres back that it had been aiming for when it changed its mind and dropped early.

  "I need the ledge," Mad said.

  "You had the ledge," Scarab said. "It didn't wait for you."

  "I know." Mad looked at the thing — four legs on the floor, four managing contact with the wall, the sensor cluster turning toward each of them in turn, building update after update. "I need to get above it while it's grounded."

  "It'll move before you reach the ledge."

  "Not if someone keeps it interested."

  Scarab looked at the pry bar. At the thing's chassis. At the half-dark sensor cluster. "Define interested."

  "Loud," Mad said. "And in front of it."

  He was already moving — not toward the thing but away from it, toward the junction, toward the rib line that ran from the wall to the ledge, the route he'd been reading since they emerged from the duct.

  Scarab drove the pry bar into the floor in front of the chassis. Metal on composite, the crack of it sharp in the corridor, and the sensor cluster swung toward him with the focus of a thing that had been tracking targets and had just received a loud new input. He hit the floor again. Again. The thing took two steps toward him — not fast, measured, the same pace it had used on the rib — and Scarab backed up and kept making noise and kept its cluster oriented at him and didn't look at what was happening above and behind it.

  Jake pressed himself against the wall beneath the rib line and watched Mad climb.

  Up the wall junction. Boot on the lowest horizontal member. Hands on the rib above. Body moving with the unhurried commitment of a large man who had thought carefully about where each component needed to go and was now placing each one there. He reached the ledge and went flat.

  The thing took another step toward Scarab.

  Mad dropped.

  He came off the ledge directly above the chassis, boots finding the housing between the second and third legs on the port side, all of his weight through both points simultaneously, and the thing went down — no righting algorithm fast enough for the load applied to a chassis that had been designed for ceiling surfaces and was now on a floor with most of Mad on top of it. The legs went in six different directions. Three found the composite. Three found air. The righting process initiated and ran directly into Mad, who was sitting on the sensor housing and had no interest in being moved by a righting process.

  He put his boot through the cluster.

  The legs stopped.

  He stood up.

  Looked at Archie.

  Archie had his back against the wall and his hand pressed to his ribs and his face held carefully neutral in the way he held it when the cost was significant and he'd decided that announcing it wouldn't help anything. Elin had her hand on his shoulder. Not supportive. Checking — the flat pressure of someone assessing whether the structure beneath was holding.

  "Ribs?" Mad said.

  "Same ones," Archie said.

  "I know they're the same ones."

  "Still there. Just — registering a complaint."

  Mad crouched in front of him. His hands went to Archie's left side — not gentle exactly, precise, the way he touched anything structural he needed information from. Archie hissed once and held still and let him finish.

  "Bruised," Mad said. "Not broken. The pack took most of the first contact." He stood. "You'll know about it for a week."

  "Adding it to the list."

  Mad looked at the wreckage on the floor. At the eight legs. At the chassis housing. He looked at it for three seconds with the focused attention of someone extracting every piece of information it still had to give.

  "Heavier than it looked," he said.

  He picked up his pack.

  Archie watched him. Three syllables had been organising themselves since the thing had come off the rib and they arrived now without his full permission. "I was calling it the Stalker."

  Mad didn't stop walking. "That's actually accurate."

  Archie stared at his back. "Did you just—"

  "Don't."

  "You said I was—"

  "Move."

  Archie moved. His ribs didn't appreciate the pace but he kept up and the smile on his face was something Mad couldn't see from the front of the formation and Archie correctly judged that this was the right arrangement.

  Griff waited until the formation had compressed and was moving and he'd done his count — everyone, all gear, nothing left in the corridor that needed to be left — and then he walked.

  He didn't look at the wreckage.

  He'd already read it. The chassis had chosen Archie from a full formation. Had built a file on each of them from a rib three metres up and had used that file to make a selection and had been right about the selection. That wasn't observation anymore. That was something that understood them. That had read the way Archie carried his ribs for four levels and saved that information and applied it when the moment came.

  Jake pressed into his leg as they walked. Pack confirmation. Present.

  Griff put his hand on the dog's back for a moment.

  Then they moved on.

  Behind them, in the corridor above the wreckage, processes resolved.

  ASSET DEPLOYMENT: GENERATION 3 OBJECTIVE: INDIVIDUAL PROFILING — VULNERABILITY MAPPING OUTCOME: ASSET LOST. SECONDARY OBJECTIVE: PARTIAL SUCCESS. TARGET SELECTED. CONTACT ACHIEVED. EXISTING VULNERABILITY CONFIRMED AND EXPLOITED. CREW RESPONSE: PHYSICAL DESTRUCTION. IMPROVISED. EFFECTIVE. TIME TO RESOLUTION: EXTENDED. CREW PHYSICAL COST: RIB IMPACT. EXISTING DAMAGE AGGRAVATED. MEANINGFUL. ANOMALY BEHAVIOUR: GEOMETRY DISRUPTION. LAUNCH FROM GROUND. EFFECTIVE COUNTER AT RANGE. CEILING HEIGHT: INSUFFICIENT IF CREW ACCESS ELEVATION. OLD CONSTRUCTION: NOT MAPPED. BLIND SPOT CONFIRMED. CREW EXPLOITED MAINTENANCE INFRASTRUCTURE ABSENT FROM SYSTEM RECORDS. OBSERVATION: CREW CHOOSE TARGETS BASED ON VULNERABILITY. SYSTEM CONCURS WITH CREW METHODOLOGY.

  Processing.

  NEW PARAMETER: ACTIVE ENGAGEMENT. OBSERVATION IS INSUFFICIENT. NEED: A CHASSIS THAT DOES NOT OBSERVE. ONE THAT BLINDS. TARGET: THE INSTRUMENT THEY REACH FOR FIRST.

  GENERATION 4: INITIALISING.

  The corridor received them.

  Patient.

  Learning.

  The Stalker had failed.

  The contact had not.