"Zainab. Zainab. Zee. Look at me. Okay, no, don't look at me, look at your arm, oh my God, look at the floor, don't look at the floor."

Seun came down the last flight of stairs in grey socks, a dressing gown printed with tiny cartoon sushi, and one earbud still in, and arrived in the lobby at the speed of a man who had been told his whole life to slow down. He stopped dead at the edge of the wreckage. Then he began to speak in lists.

"Ambulance. Landlord. Insurance. Fire, maybe fire. Is it fire? No. Brass. Okay. Wipe the, no, don't wipe anything, photos first. Photos. Who has photos? Is that Mr Anand from 2C? Mr Anand, go back to bed, it's fine, nobody's dead."

"Nobody's dead," Zee agreed, from the bottom stair, where Gus had insisted she sit and where she was letting him hold a clean tea towel against her arm with enormous gravity. Her left hand was fine. Her left hand was somebody else's now, lent by an agency. The cut ran the length of her forearm and it was not deep, she had counted. It looked terrible.

Seun crouched in front of her and took her face in his hands, which he did to everyone, and which she let him do because he had once sat with her for six hours in a hospital in the first month she'd come here and never said a word about why.

"You jumped over the desk," he whispered.

"The desk was in the way."

"You jumped over a desk like a, like a ninja cat. Gus says you just knew."

"I had a feeling."

"A feeling." Seun looked at her for one long second, with his round, kind, over-promising face, and she watched him decide not to ask. It was one of the things she loved about him and one of the things that would eventually get someone killed. "Okay. Feeling. Great. I'll tell them corrosion. Brass gets, you know. Tired."

"Brass gets tired," said Gus, nodding. "That's so true."

By three, the lobby had filled. People came down in the clothes they had fallen asleep in and stood at the tape of the unspoken rule that a disaster is for looking at, not helping. Mrs Nyambura-Voss from 6C in a headscarf and a face of pure fury at the existence of Tuesday. A man in 2B in a full cycling kit for reasons nobody asked. Two Brazilian developers in matching pyjamas. A woman Zee only knew as the cello, because of the sound of her, who looked at the folded desk and said, quite calmly, "It would have killed someone," and was ignored.

Zee heard it anyway. She sat on the stair and watched the chain that had not snapped hang loose from the ceiling, swinging slightly, the other two ends dangling in a small iron wind.

Something was wrong with the way she was sitting. She had been trying to work out what for several minutes and now she had it: she was counting wrong. All her life, since the age of nine and her father's stopwatch, she had carried a clock in her chest that told her without effort how long she had been somewhere. And it had slipped. There was a gap in it, a patch of nothing. She did not know what time it was.

She looked at her phone. 3:06. Fifteen minutes since the crash. She would have said eight.

Then the front door opened, and she saw the man.

He came in out of the damp street the way the footage had said he would. A tall man in a long dark raincoat, mid-thirties, with a rucksack on one shoulder and a pair of big old studio headphones slung around his neck. He stopped in the doorway, because everybody stops in the doorway of a crash, and the light from the lobby caught his face. It was long and tired and kind, with a day of stubble and heavy glasses that had slid down his nose, and he was looking not at the wreck but past it, past the crowd, and the instant his eyes found her they stopped searching.

He crossed the lobby, stepping over a splinter of brass the way you step over a sleeping dog. He stood in front of her. She felt her whole body make a decision to stand, and then not stand, and then she understood that she knew exactly what he was going to say. She had read it off a ceiling camera.

"Hello, Zainab," said the man.

Seun said, "Oh, hi, Rui! Isn't this mad? Zee's fine, a thing fell."

"I can see that," said the man, and Rui turned his head a little to the right, the way you turn a good ear toward someone. "I'm sorry. That was an odd thing to say. I heard the crash from the street, I think half the harbour did. Are you hurt? Is that your arm?"

"It's a scratch."

"It's a long scratch." He crouched, glasses sliding, and looked at the tea towel with the unembarrassed interest of a person who spent his life looking at damage to things. "Do you have anyone to stitch it?"

"I have a kit."

"That's not an answer. That's a list of things you own."

It was such an accurate hit that she nearly laughed. Seun gave a tiny sound like a kettle.

"Rui lives in 4B," he said, to the air in general. "Rui is our audio guy. He fixes old tapes. He's very quiet. The only complaints about him are that he's too quiet."

"There was one about the cheese," said Rui.

"That was not a complaint. That was concern."

She stared at him. Four months, he'd been in the building, and in four months she'd seen him four or five times, in the lift, with the headphones, giving the little nod of the very shy. Notes under her door about the lift. Neat handwriting. Signed R. She'd thought nothing of him. She had a whole file in her head for people who thought nothing of her, and he had been in it, under harmless.

"How do you know my name?"

He took a moment. "I've read the notice by the lift," he said. "Concierge, Z. Oyelaran-Pereira. It's on the brass plate."

"My first name is Zee on the plate."

"Then I've said it wrong," said Rui, and she saw him decide to lie to her and saw that he was bad at it. "And then, you know. The other thing. Everyone knows the other thing. I'm sorry. I didn't say anything because it was none of my business. It still isn't. I'd just rather not pretend I didn't know, if you're going to look at me like that."

"Like what?"

"Like I'm a man in a clip."

The lobby had gone on talking around them. She heard only the hum of the lift and her own pulse with its odd stutter. She had told no one. She could not tell Seun, who would put it on a rota. Could not tell Gus, for whom everything was a feature of the product. And she could not stand being the only person in a building of forty-one flats and a hundred and twenty tenants who had seen what she had seen.

She took out her phone.

"Rui. Come here. Don't say anything. Can you watch something for me?"

She turned the screen toward him. It was the Loop page, paused at the white-lettered caption. She watched him read it and watched his face do the thing everyone's face did, the wince, the pity, the sorry-sorry, and she held up her good hand to stop it.

"Not that. The end. The part after. Just tell me if you see it."

He took the phone. She saw him slide the headphones up over his good ear and clamp the left one against the side of his head, the dead side, the way a man holds a shell to listen. It was a gesture so private she looked away.

"It's thirty-four seconds," he said.

"Press play."

He pressed. The tiny speaker clicked and the picture ran. She did not need to watch. She watched his face instead, the soft pale light of the screen on his glasses. The terrace, the rain, the knife. She saw him tense at the exact second the film should have frozen, saw his eyes narrow as it did not, saw him look from the phone to the ceiling of the lobby, to the camera in its dome, to the dangling chain, and back.

"The picture goes on," he said quietly. "It's here. This is here. This is the lobby."

"Yes."

"This is tonight. This is, God." He lifted his eyes. "This is the man from three. This is you. There's you on the desk. Zainab, this was before it happened, wasn't it? This was playing before?"

"Yes."

"That's not possible."

"It's what happened to my arm."

He swallowed. He did not say it was a prank. He did not say deepfake. He went on watching, with his head tilted and his face gone absorbed in the flat way of a person who has been trained for years to listen to nothing and find a thing inside it. At the point where she crossed the desk he made a small sound. At the end of the footage, when the figure in the raincoat looked up into the lens, he stopped breathing.

"That's me," he said.

"Yes."

"I came in. I said that. Just now, I said those words, and I didn't, I didn't plan to." He pushed his glasses up. His hand was not quite steady. "But that's not what I wanted to tell you. Do you see this thing here, the waveform? Under the video?"

"There's no sound."

"There is, though. That's the strange part. There's something in the sound track. It's very low. It's so low most phones can't make it." He glanced at the crowd, at the lift, at Seun being heroic to a cyclist. "Do you have headphones?"

"In my coat."

"Give me a moment and the good ones."

She got her earbuds from the desk drawer in the wreck of the desk, and he put one of his own studio cans against his good ear and one of hers against hers and sat beside her on the cold bottom stair, a thin man in a damp coat with his knee touching hers, and started the thing again. Without meaning to she leaned in. They shared the screen. They shared a wire. In the tiny speaker, over a hiss of nothing, she heard it.

A room. A room's dark, lived-in hush, the sound of a space with people in it who were not saying anything. The creak of a chain. Then, very close, so close she felt it on the skin behind her ear, someone's breath. A slow, wet breath, drawn in through the nose, held, and let out again, patient, like a person in the front row of a theatre who has been waiting for the curtain.

It was not Rui. Rui was beside her, mouth shut, eyes on her face.

It was not her.

It was somebody standing in the lobby of Casa Alvorada, very close to the camera, in the eleven seconds that had not happened yet.

And in the last second of the file, as the figure in the raincoat finished saying her name, the breath leaned in and whispered one word she could not make out, one flat syllable, as though it had been waiting two years to be allowed.

"Again," said Rui, very quietly. "It says again."