Casa Alvorada's fourth floor smelled of toast and solder.

Rui's flat was 4B, at the end of a corridor whose carpet had been the colour of old wine before three owners of the building had walked it into the colour of old pavement. He let her in at ten to seven with a hand still pressed to the side of his head, as if he had forgotten it was there, and she stopped three steps into the room because she had never seen a flat so completely arranged around a single idea.

Everything was either a machine for listening or a thing that stopped sound from escaping. Foam panels in grey wedges on every wall. A door with a rubber seal like a fridge. Along one side, shelves, and on the shelves, tape machines: reel-to-reel decks the size of suitcases, cassette decks like small black bricks, a wooden thing with a brass horn that had to be a hundred years old. In the middle, a desk with three screens. In the corner, a narrow bed with a duvet in a startling shade of orange, and above it, a framed postcard of a lighthouse with the caption MIND THE GAP in green marker.

On the kitchenette counter sat a block of cheese under a glass dome.

"That's it," said Zee. "That's the cheese."

"It's not a smell. It's a character." Rui took her coat from her as though that were a normal thing to do in a flat. "It's a washed rind, they ferment it in brine, it's completely safe. Tea?"

"Do you have anything not alive?"

"I have instant coffee that's technically dead."

"Perfect."

She sat on the one chair that was not a speaker. Her arm had been stitched an hour earlier, in a pop-up clinic of one: Seun's cousin, a nurse, in her pyjamas, with strong opinions about brass. It ached with a long, patient ache, like a thing being slowly explained to her. Under the ache something stranger had begun. She kept losing small amounts of time. She would be looking at the foam on the wall and then the foam would be a different shape, and the clock on his desk had jumped. It was not sleep. It felt like a lift going past a floor.

"You're counting," Rui said, handing her the mug.

"Pardon?"

"Your lips. You move them. Every few seconds. I've been watching you do it since the stairs." He sat down on the floor with his back against a speaker, folding his long body in sections. "I don't mean it as a criticism. I do it too. It's a thing people do who've spent their lives with waveforms. Everything comes in units."

"It's an old habit."

"Good habits are always old." He opened a laptop on his knee. "So, I'd like to propose we do this properly. I think we're both frightened and I think neither of us wants to say so, which is how people end up in cupboards in films. Controlled conditions. One variable at a time. Let's find out what it does."

"What it does," said Zee, "is kill people."

"It saved one."

That stopped her. She turned the mug a quarter turn on her knee. The coffee was awful. It was hot and it was the exact thing she needed.

"Fine," she said. "Rules. Tell me what you want to know."

He wanted to know four things and he had a notebook for it, a real paper one, and a pencil, and as he wrote she saw that he printed with a draughtsman's capitals. First: whether the clip added the same length every time. Second: whether it showed only the place where it was played. Third: whether the footage was a fixed record or a forecast, because the difference mattered, he said, in the way the difference between a photograph and a weather report mattered. Fourth: whether it could be copied.

"Why copied?"

"Because if it could be copied," said Rui, "someone else could have it. And I would rather like to know who else is on this list."

They did it all in the order he chose. The phone, first, on the desk, propped upright against a monitor, so the lens of its camera was blind and the screen played to the room. He set up a reference microphone, absurdly large, on a stand, as if recording a visiting dignitary. They listened to the sound again, the long hush, the breath, the one-word whisper. It was less frightening in a studio. It still lifted the hair on her arms.

"Start," said Rui.

She pressed play with a little one-handed flourish she did not feel.

The terrace again, the rain, the knife. She had developed a technique over the last hours of looking at the wall during those thirty-four seconds. It was a bad one. You could not see yourself fail without seeing it. Then the lobby, the can, the chain, the sunburst, the tidy ruin. Now there was the clip's own extra, the dark doorway and the man in the coat, and Rui watched himself arrive and said "I look dreadful" in a small voice. And then the picture went on.

The counter said 1:07.

The picture was a ceiling again, but higher and flatter, and she realised it was the top of the shelves at the far end of the room, a camera's-eye view looking down at the desk, the chair, the orange duvet, herself. In the picture it was exactly this morning. A woman with a sleeve rolled to the elbow, sitting with a mug on her knee. A man on the floor against a speaker, with a notebook.

"Eleven seconds," said Rui, writing. "Count."

"It's the same count. I make it eleven."

At 1:11 on the little counter, in the picture, Rui reached for the pencil jar on the desk and knocked the mug off the edge. The one beside the laptop. A big white one, with a chipped lip, and a handful of cold dregs. It fell. It hit the floorboards. The picture caught the pieces going out across the boards like a spilled hand of cards. And the man in the picture, reaching down for them, laid his bare palm flat on the biggest piece, and a dark line opened across it.

"Pause," said Rui, quickly. He didn't need to, the clip was still ahead of them in time. He just wanted to say it. "Pause it. It's the cup. I'll move the cup."

"No."

"No?"

"Variable three. Is it a fixed record or a forecast." She had a feeling like a runner sliding into position. It was, without a doubt, the first time in two years she had felt good. "Leave the cup. I'll catch it."

"You'll what?"

"I'll catch it. Hit play from where it stopped. I'll tell you if it changes."

It was so absurd he laughed, a single short surprised bark, and then did exactly what she said. The picture played on. She watched herself, in grey, watch Rui. She counted. Ten. Eleven. As the cup left the desk in the little footage she moved, not even properly thinking about it, in the old flat way, left foot, hip, a sweeping low reach from the chair, and in the real room the mug dropped into her palm like a coin from a magician's sleeve. A small cold slop of coffee on her wrist. Rui stared up at her with the pencil raised.

"You caught it."

"That's what I said I'd do."

"I didn't, I hadn't even, I hadn't knocked it yet." He put the pencil down with exaggerated care. "I hadn't knocked it. You'd done it before I knocked it. You'd done it while I was still reaching."

"Play it again."

"We should save the number of plays."

"Rui."

"Right. Yes."

He did it, hitting the key with two fingers like a man defusing something. The terrace, the rain, the lobby, the doorway. The shelves. And then the room, and the eleven seconds before them, only this time the film ran the way it had run in the air: Rui reaching for a pencil, Zee out of her chair with a whip of the arm, the mug in her palm. In the footage the man on the floor said, "You caught it," in pale silent lips. The counter said 1:18.

"It's a forecast," said Rui, hoarsely. "And then it's a record. The first time you watch, it shows what's going to happen. If you do nothing. And then it, what, it rewrites? It edits itself to show what you did."

"Draft and final cut," said Zee. "That's what an editor would say, isn't it? You told me you were one."

"I fix old tape. I'm not an editor, I'm a restorer." But he wrote it down. He wrote DRAFT / FINAL in capitals, underlined twice. And under it, more slowly: ELEVEN SECONDS EACH PLAY. ONLY THE ROOM IT'S IN. And a small question mark beside CAN YOU CHANGE IT, and then he crossed out the question mark and put a tick.

She should have been triumphant. A forecast she could fight. Instead a thought slid under her ribs that she did not like. When she'd caught the cup she had gone away. Not far, only for the space of one breath, a pocket of nothing between the reach and the catch. Her reflection in the dark monitor had been late again. The woman in the glass reached for the cup a beat behind her, like a bad dub.

"Zainab."

"Zee."

"Zee. You've gone grey." He had half risen. "Your lips. They've, you've lost, sorry, I'm not trying to make a thing of it, you're a little bit grey."

"I'm tired."

"Would you like to be tired in a chair?"

"I'm sitting in a chair."

"Then be more tired in it. Please."

She let him bring the duvet, for no reason except that it was orange and ridiculous and it made her smile. She sat inside it with the clip on her knee while he ran his last test, which was to copy the file to his laptop, to see whether the copy grew, and the copy, when he played it, did: it added its own eleven seconds, the same eleven seconds, a perfect twin of what the phone had shown. Both devices said 1:18. Then he played the phone's again to test, and the counter on the phone moved to 1:29, and so did the copy, without being touched, without even being open.

"It's one thing," said Rui. His voice had gone very quiet. "It's not two files. It's one thing wearing two coats."

"Rui, do you have a way to delete it?"

"I could try."

"Try everything. Later. First finish the last variable."

"It's not a variable. I didn't write it. I want to look at the room."

He pulled up the footage again, the last eleven seconds, from 1:18 to 1:29, the ones that had been added by the second play. They had not looked at them properly. They had been too busy being clever.

It was the room. The shelves. The orange duvet. Zee in it, pale, her knees up. Rui, at the desk. In the picture the corner of the table lamp made a halo on the foam wall and on the shelf above, beside the cassette decks, sat a small flat cylinder of white plastic with a thin ring of light round its edge. A Hearthwell Nook puck. The kind the building's management had fitted in every flat last winter, so that you could ask for the weather in a pleasant voice.

The ring was glowing.

In the picture, as the eleven seconds ran out, the puck turned very slightly on its shelf. As if toward the camera. As if it had heard its name.

Rui looked at the real shelf. They both did. The same shelf, the same decks, and the same small white cylinder with its flat, grey, blind face.

"I unplugged that in March," said Rui. "I took out the plug. I put it in a drawer, and then I put it back, because management does spot checks, and I took out the cable at the wall."

"It's not on."

"It's not on," Rui agreed. "It's not plugged in. It's dark. It's physically impossible."

On the shelf, the puck's ring was dark. On the screen, in the last frame of the clip, as the counter ticked over and the picture froze, the little ring of light lingered on its blind face and then, like an eye closing, slowly went out.