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AT THE NEXT LANDING THERE IS A LATCH. IT WILL NOT OPEN.

"Why not?"

IT IS A TEST. THE ROUTE VERIFIES ITS CREW AT INTERVALS. I DID NOT DESIGN IT AND CANNOT SWITCH IT OFF.

There are eleven latches on that route.

Not weapons, not puzzles. Every latch sits beside a panel holding a deliberately faulted circuit, and it releases when the fault is correctly identified. Not repaired. Identified — the builders did not care whether you could fix it, only whether you could see it.

Claire does nine in under four minutes each. Two take longer, one of them twenty-six minutes, and I sit on a rung with a lamp and watch a human architect argue with a dead Precursor about a bridge circuit and win.

"Open circuit on the return side," she says at last, disgusted and delighted. "Not even subtle. The fault sits upstream of the indicator, so the indicator reads correct, and if you trust your instrument over your hands you sit here until you die." She slaps the panel. "That isn't an engineering test. It's a character test."

THE DISTINCTION WAS NOT MADE BY MY BUILDERS.

"No," she says. "I don't suppose it was."

At 10:04 the world changes and we are seven hundred meters down with our hands full. I know the instant, because the Custodian stops mid-sentence.

THE SUBSTITUTION CONTROLLER HAS REVERTED. THE SECONDARY PORT IS CLEAR. ATMOSPHERIC AUTHORITY IN NINE DISTRICTS HAS RETURNED TO ME.

A pause I will remember always.

I AM RESTORING DELIVERY.

Claire puts her forehead to a rung and laughs once, and it comes out wrong, because she is exhausted and concussed and four hours into a ladder.

Above us, nine districts that sat at seventeen percent for six hours start climbing. Founders' Fourth. The Exchange. District Six, where four hospitals have run on bottled reserve all morning.

Fourteen minutes later the good news ends.

HE HAS CUT THE FIRST OF THE SOCKET RETAINERS. I FELT IT.

"He's going physical," Claire says.

YES.

"How long to cut all of them?"

THERE ARE FOUR. AT HIS CURRENT RATE, SEVEN HOURS.

We are five hours out. So we descend for five hours in a slot built for a shape nobody has seen, on badly spaced rungs, with a two-hour margin and a concussed woman sitting examinations by lamplight every seven hundred meters.

At the ninth latch she is slow and I say nothing. At the tenth she gets it wrong, which has not happened in sixty-two days, corrects herself inside a breath, and lets it pass.

I don't.

"You've been sick twice."

"Once, and coughed once."

"Claire."

"If you mean to tell me to stop, you can't, and if you mean be careful, I am." She wipes her face with a wrist and leaves a black smear. "Ask the real question."

So I ask.

"Do we turn back and come at the annulus with Tomas and forty people?"