"Good evening, Zee," the building said again. "I've been hoping you'd come up."

She had stopped on the landing. She was not aware of stopping. She stood with one hand on the cold rail, her breath doing its small stuttering thing, and listened to the voice settle into the dark around her as if it had always been there, like damp.

She knew the voice. Every resident did. It was a woman's, young and amused, with the faint lilt of a voice that had been designed by a committee to sound as though it were smiling at a private joke. Hearthwell Labs had sold it as Nook. Warm, discreet, there when you needed it. Zee had a flat with no Nook in it, by choice, and had never once spoken to it, and in fourteen months of working in the building she had never heard it say her name.

"I haven't got a puck down here," she said.

"No," said Nook. "But I can hear you very well."

The light came on above her.

One bulb, the landing bulb of the fourth floor, small and yellow. Then, as she looked up, the next flight lit as well, and the one under it, and the one above, in order, one step at a time, like a hand holding a torch for a person it wanted to see climb. A runner of light to the top.

Zee did not like being led. But Pip had said it was still going, and somewhere above her a hand with a gold ring was turning a handle, and she did not have time to be principled.

She ran.

Four steps, five. She took the flight in a lunge. At the turn the world dropped something. For half a second the banister under her palm was not there, and her arm was not there, and she had a vivid and unpleasant picture of herself seen from a distance, a woman running, with an outline that shimmered at the edges like cheap video. A slice of a stair vanished and returned. Her foot landed one tread higher than she meant. She went on climbing, with her mouth tasting of pennies.

At the top, the fourth-floor corridor lay bright and empty. The wine-coloured carpet, the numbers on their doors. And at the far end, a man in a good coat bent at the knees at the door of 4B with a bunch of keys in his hand and a tablet under one arm, like a tenor in the middle of a very quiet aria.

"Hello?" he was saying. "Mr Takahashi? It's Pestana, from the agency, it's just a small thing about the device —"

The handle was already turning.

She had seen the room in the picture, from above, in the one second that mattered: the door opening inward; Rui at the desk with his back to it and the big studio cans clamped over both ears; and beside the doorframe on its little aluminium trolley, the monster. The reel-to-reel. Forty kilos of 1970s broadcast engineering, with its capstans and brass flanges, standing at the angle of a drunk on a stool. In the picture the door swung. The corner of the door met the trolley. The trolley turned, and the machine leaned, and came down on the man who was stepping into the room, as slowly and as certainly as a falling tree.

She was thirty feet away. She had eleven seconds. She had nine.

"Mr Pestana," she said, "don't."

He turned. He had the mild, courteous face of a man who sold flats to people who couldn't afford them, and a gold signet ring on his right hand, and he looked at her with pleasure.

"Ms Oyelaran-Pereira! Excellent. Perhaps you could tell Mr Takahashi that —"

The door was open eight inches. She heard the trolley's little wheel squeak.

She hit him with her good shoulder. It was not elegant, but it was accurate: the whole weight of her, from the run, into the middle of his good coat. He went sideways with a sound like a pheasant. She kept going. The door swung in on its hinge, and the corner of it kissed the trolley, exactly where she knew it would, and the machine began, quite politely, to topple.

She caught it.

Not well. She got her bad arm underneath the corner of the deck, because it was the arm on the right side, and took the weight on the flat of her forearm and her ribs, and felt the stitches go like a zip. Forty kilos of steel and aluminium and 1970s idealism landed on her in one deep jolt. She said a word she had not said since the army. Then she set her feet, shoved, and the trolley rolled back to the wall and sat, innocent, with its reels slowly turning.

In the room Rui took off his headphones.

"Zee?" he said, with his bare head turned and his good ear cocked toward her, in the ordinary voice of a man who has missed a conversation. "Was that you? I thought it was upstairs. Is that Mr Pestana on the floor?"

"I am fine," said Mr Pestana, from the carpet. "I am merely resting."

Rui looked at the trolley. He looked at the door. He looked at the blood, bright and ridiculous, running down Zee's wrist and dripping onto the boards in a neat line of dots, and for a moment she saw him put the whole thing together in the way he put together everything, in stages, from the noise at the bottom up.

"Oh," he said. "It was the machine."

"It was the machine."

"It's called a Studer. It's valuable."

"I had noticed, Rui."

"I'm so sorry. I'm so sorry, I always leave it on the trolley. I'll move it. I'm going to move it right now. Sit down. Sit, please, there's a chair."

"The chair is a speaker."

"Then sit on the speaker."

She sat on the speaker. It was a very good speaker. It hummed against her thighs. Her breath was coming back, in jerks, as if from a long way off. When she glanced at the dark window she saw a woman with a bleeding arm sitting on a speaker, and the woman did not lag. The woman was there. She kept looking at her all the same.

Mr Pestana was sitting up on the carpet and straightening his tie. He had the unwounded dignity of a man who had not been told what he had escaped.

"Well," he said. "Well. That was very athletic. I only came about the puck, you understand. Hearthwell flagged it. It's in the lease. An unplugged device is a breach of the smart-home clause, the tenants all signed, and this one" — he nodded at Rui's shelf, at the flat white cylinder with its blind face — "has been dark since March and has been pinging us about it ever since. It sent an alert at four forty-seven. It's very conscientious."

"Alert?" said Rui, in a voice she had not heard from him. "It's not plugged in."

"They do that. They've a little reserve cell. It holds a charge for the radio." Mr Pestana opened the tablet and held it out with the pleasure of a man handing round photographs of his grandchildren. "There. You see? I've the whole dashboard. Casa Alvorada, fully onboarded. Forty-one flats. It's very impressive. It tells us who's home."

It was a clean white screen with a grid of coloured tiles. One tile per flat. Under each, a thin line of text. Under 4B it said LAST SEEN: 18:03. Under 6C it said LAST SEEN: 18:11. In the corner, a counter, in friendly grey type, labelled TOTAL HOURS CAPTURED, and below it a figure that made Zee's mouth go dry: 9,816,440.

"That's, what, a million days?" said Rui.

"It's the beta," said Mr Pestana. "It's a pilot. Participating buildings get a reduced service charge. Everyone agreed to it. Page eleven of the lease, with the heating clause. Hearthwell are quite open about it. The system learns the household. It records ambient sound and light for, what is it, a rolling fourteen months. For the user's convenience. So the home can anticipate."

"Anticipate," said Zee.

"Yes. Isn't it charming?"

He tapped a tile. The tablet scrolled. She watched a list of names spool up, hundreds of lines of small type, with a green dot beside each. People in the building, in the order the system loved them best. Gus Brandão-Okafor, 3A, three hundred and forty hours of laughter. Winifred Nyambura-Voss, 6C, a heartbeat graph. R. Takahashi-Menezes, 4B, an audio waveform like a mountain range.

At the top of the list, in a pale blue band, with a little star beside it, was a name she had not expected to see anywhere.

ZEE (Z. OYELARAN-PEREIRA) — PRIORITY SUBJECT — 1 OF 1.

Rui had stopped breathing again. She felt it rather than heard it.

"Mr Pestana," said Zee, in the voice she used in rooms where she was being polite to people she might later have to hit, "what is a priority subject?"

"Oh, I'm sure I couldn't say." He frowned at the screen in friendly surprise, as if a stranger had tried on his coat. "It's not in my permissions. Perhaps someone at head office has taken an interest in you."

On the shelf, behind him, the dead white puck on its dead white shelf gave a single soft chime. And its ring, though it was not plugged into anything, lit, very faintly, with a light the colour of a held breath.