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The gallery's inner wall carries load on ribs at four-meter spacing, and between them the panel is facing, not structure. I know where every rib is, because a Precursor rib sits at the fourth seam. Cut the facing between two of them from behind and eleven meters of walkway loses what it stands on, and the men on it drop into the interstice with nothing structural harmed.

I do that twice. The third position needs nothing from me, because by then the Custodian has grasped my method.

IT IS COLD DOWN HERE, it says, from a dead lens, running on the last of nothing. I HAVE NO POWER FOR DOORS. I HAVE PRESSURE.

"Take it from the drop side."

YES.

It cannot open a bulkhead. It can open a relief valve, which costs nothing, and two atmospheres of differential across a ring gallery moves an enormous volume of air through the one gap available. Nobody is hurt. Eleven men go down, every lamp blows out, and the arc fills with six centuries of dust, and I come through the middle of it, and I know exactly where I am going because the interstice needs no light.

Then something else happens, and it happens to them and not to me.

The controller starts failing.

I don't know what she has done. I know their confidence changes around 22:30 — shouting below, a man sent up the drop ladder twice, the particular tone of people finding that a finished thing is not finished. Later I learn she had booked the whole transplant in as a fourteen-hour maintenance task, and that the Custodian, being ancient and thoroughly procedural, had already scheduled its own reversion and begun, in the tidy manner of maintenance systems everywhere, to prepare for it. Pre-staging the old configuration. Reserving the socket. Refusing three attempts to make the change permanent, because permanent alterations need a second signature and there wasn't one.

Marr's pair spend an hour trying to promote a work order they cannot read the class field of.

It is the funniest report I have ever been given, and I did not laugh at the time.

I come onto the lower gallery at 23:11 through a panel nobody has opened since this place was built.

Nine men in disarray, a controller half-seated in a socket politely declining it, and Claire lashed to a work stool with her chin up, arguing.

She sees me over Marr's shoulder. She shows nothing whatever, which is how I know she is entirely herself.

After that, eleven seconds I will not describe, except to say I killed nobody, because she asked me for paperwork under a tram stop three weeks ago and I have thought about it daily since.

Marr goes up the western drop ladder with four of his people and the controller under his arm before I get across the floor. I let him go, because Claire is tied to a stool and no part of me chooses the ladder.

I get the cord off her wrists. Her hands are a mess, there is blood in her hair, she is gray with concussion, and she is furious about none of it.

"Fourteen hours," she says.

"Claire—"

"Fourteen hours from 20:04 and it reverts on its own. We just need to be alive and the socket needs to exist." She grips my forearm hard enough to hurt. "Are you injured?"

"No."

"You're lying. I can feel it."

"I am hurt, it doesn't matter, and if you say the word concussion before you sit down I will carry you."

She laughs and then stops laughing because it hurts, and puts her forehead against my chest for about four seconds, which is all we have.

Because the gallery is beginning to complain.

Nineteen hours without cooling or assistance. Every thermal barrier in the annulus is off, the rock is at seventy-eight degrees and climbing, three expansion joints have gone past design travel, and below us a structure never once allowed to run hot in ninety centuries is finding out what that feels like.

A section of the fifth ring lets go while we are still standing there.

"We need to be clear of the annulus," Claire says. "Now. Then we work out how he reaches the core, because he can't authenticate layer six, and he knows that."

"He has the controller."

"And four hours before it becomes a paperweight, and he is out of clever options." She gets up, sways, refuses my arm on principle for a second and a half, then takes it. "He's going to force it."