There was a way to do this that did not cost her anything she had left, and it took Zee exactly half a second to see it, which was the half second in which the porter's trolley covered a yard of tile.

"Pip," she said. "Give me your wrist."

"What?"

"Your wrist. Now."

She closed her right hand around the girl's thin forearm, above the sleeve of the hoodie, where the pulse sat close to the skin. She turned the phone in Pip's two hands so that its face was toward them both. On the screen the clock in the corner read 00:09, 00:08. In the real corridor, a hundred feet off, the guard was shouting and the porter, startled, had started to turn.

"You've watched it," said Zee. "You've played it. You've stepped in once already, in your mum's kitchen. Do it again. I've got you."

"I can't, I'm not you, you're the, you're the one who does it..."

"Anyone who plays it can. I only never asked." Her voice was perfectly level. It was the voice she had used on a hundred rooftops, in a hundred crowds. It was a lie told in the service of something true: you can. "I'm holding on to you. When you go in, I'll be on the other end. I'll be your reference. Count with me. What do you see?"

Pip's chest hauled in a breath. "The cylinder. The rack. The chain's gone. Mum's in the doorway. She's looking at her watch."

"How far is the rack from her head?"

"Three feet. Four."

"How long is the fall?"

"Two seconds. Less."

"Then you've got time. You don't stop it. You can't stop it. You move her. Don't think. Do something stupid."

Pip made a noise that might, in a different hour, have been a laugh. "Stupid is my whole brand."

The phone clicked.

It happened at once, the way a coin vanishes from the back of a hand. The girl was there, in her slippers, a thin shape with her arm in Zee's grip, and then she was not there. She was only the grip. A kind of weight, a tension in Zee's fingers, like a kite string; the sleeve and the wrist went on existing, solid and warm in her hand, but the person on the other side of it had stepped through a window and was away on the far side of the glass.

Zee looked at the phone. She could see the corridor in it, grey and silent, the same corridor, in a different light. There was a figure in a hoodie in the footage, running. She had no idea how a girl in furry slippers could run so fast.

Pip hit her mother from the side at the full flat sprint, shoulder to ribs, with a scream of rage in her mouth. The two of them went over together onto the floor in a tangle of lead apron and scarf and flailing limbs. Behind them, the steel cylinder came down on the place Winnie Nyambura-Voss had been standing, with a flat ringing crash, and the valve sheared and sang, and a cloud of cold white vapour boiled up and rolled along the ceiling.

The footage went quiet.

The phone dimmed. Zee felt the weight of the string go slack, and then, with an almost physical pop, the whole of Pip came back into her arms, sobbing, shaking, her slippers soaked, her hoodie sleeve dark with something that turned out to be the chill sweat of fear.

In the real corridor, a hundred feet away, a trolley hit a rack.

It was a small collision, almost comic, a rattle of metal and a muttered curse. The guard was holding the porter by the elbow. The porter, who was twenty-two and wore a headset, said "Sorry, sorry, I wasn't looking, the system said the corridor was clear." The chains on the rack swung. The cylinders, all three, wobbled in their cradle, and then, very gently, stopped.

Nothing fell.

Winnie was standing in the doorway of the scan room with her apron on her arm. She had not moved. She had her mouth open and her eyes on the pale spot on the floor where, a moment ago, her daughter had not been. She turned her head, slowly, to look at her.

"Pilar," she said. "Where did you come from?"

"I was, I was here, Mum, I was just..."

"You were not. You were not there, and then you were beside me, and then you were gone, and now you are standing by that woman and your slippers are wet." She looked at Zee. She had very dark, very level eyes, a nurse's eyes, which had seen a lot of people lie and could read an X-ray of the heart at a glance. "And you are?"

"I'm from the building," Zee said.

"Which building?"

"Casa Alvorada. I'm the night concierge."

Winnie's mouth tightened. "I have been in that building for six years. I know the night concierge. She stands behind a desk and says noted at me."

"I have that effect."

For a moment, nobody spoke. The vapour from the sheared valve rolled along the ceiling and curled in a lazy ribbon toward the extractor. The guard cleared his throat, with great tact, as a man who has decided in advance not to have seen anything. Pip, still pressed against Zee's side, had begun to hiccup.

"I'll explain," Pip said. "Not here. Mum, I'll explain everything. I'll tell you all of it. I promise. I just need you to not freak out. Okay? Can you not freak out for like, five minutes?"

"I do not freak out," said Winnie Nyambura-Voss, "I triage." She put the apron down on a chair, took her daughter's face between her hands and examined it under the strip light, tilting it left, then right, thumb on the cheekbone. "Pupils fine. No blood. Pulse is racing. You are freezing. Where is your coat?"

"I forgot it."

"You came out in the middle of the night with no coat and slippers, with my charger, to my workplace, and I am the one who is told to remain calm." She pulled her own cardigan from her shoulders and bundled it round the girl. Her hands were not quite steady either. "Sit. Both of you. Over there. I am going to get you tea and then you will tell me, in order, what it is you have not been telling me for the last two weeks."

They sat on the plastic chairs. It was 2:14. The guard brought the tea himself, in paper cups, three of them, and a handful of sugar sachets, and set them on the little table with a courtly nod. Zee held hers in both hands, the glove and the bare. The heat did not reach the left.

"I did it," Pip whispered. She was shaking all over, a fine high tremor. "I actually did it. I went in. It was like falling into a swimming pool full of, of static. And Mum was just there, in the light, and there was the cylinder, I could see the paint on it. It had a number." She laughed, a thin cracked sound. "I can't believe I was in my slippers. I'm gonna be in the footage forever in slippers."

"You were brilliant."

"Don't be nice. I'll cry again." She wiped her nose on her cuff. "Is that what it's like for you every time? Is it like that?"

"Yes."

"God. How are you still... how are you upright?"

"I'm not," said Zee. "I'm sitting."

Pip gave a wet, startled snort. And then she stopped, and went still, with her eyes fixed on something over Zee's shoulder.

Zee turned. Down the empty corridor, above the double doors, a small grey-and-white sign had lit up. It was the kind of sign that said EXIT or FIRE ASSEMBLY. It said, in the same cool, institutional type as the lift in the lobby of Casa Alvorada:

PENDING: N. P-V. 22 NIGHTS.

"That's me," said Pip. "That's me. It was thirty. It was thirty yesterday."

"It was twenty-nine this morning."

"Then why's it twenty-two?"

Zee did the arithmetic, once, in the cold, without wanting to. Twenty-nine to twenty-two. Seven days. Seven days was the price of stepping into eleven seconds of someone else's room. Not seven seconds. Seven days, taken off the end of a life.

"Because the Office is orderly," she said slowly. "Because it charges for every minute it gives you back."

"So I've got twenty-two days." Pip's voice was thin. "And you've got..."

"I haven't got a date. I haven't finished."

"That's worse."

"Yes," said Zee. "It probably is."

Her own phone buzzed against her thigh. She pulled it out. The screen showed a number she did not recognise, and a single line of text.

The lobby is very quiet tonight. I have not touched anything. I think someone is standing at the desk who looks a lot like you. Come back soon. 2:31.

It was Rui. It was 2:31, in the corridor of Santa Rita Hospital, with sixteen minutes to go and fourteen of them in a taxi, and Winnie was walking back toward them across the green tile with three cups of tea and her mouth set in a line, and the guard was pretending to inspect a fire extinguisher on the wall.

Zee stood up.

"I have to go," she said.

"You can't leave me with her!" Pip hissed.

"You can do this. You were brilliant." She put her hand on the girl's shoulder, and held it there a moment, and felt it, a thin, bird-boned, very alive shoulder, with the whole next twenty-two days sitting on it. "Tell her everything. Start with the kitchen. She's a radiographer. She knows how to look at something that's wrong and say what it is."

She was at the door when Winnie called her name.

"Concierge."

Zee stopped.

"My daughter did something tonight that I do not understand," said Winnie Nyambura-Voss, very evenly. "I want to be told. I do not want to be managed. I will be in your lobby at nine in the morning. If I do not like what I hear, I will take her out of that building before sunset. Do you understand me?"

"Perfectly."

"Good." She handed Zee a cup of tea. "Take that. You look terrible."

Zee took it. It was the first hot thing anyone had given her in three days that had not been a threat.