Zee woke in the staff room with her heart going like a fist on a door.

For a moment she did not know which door. The room was seven feet by nine: a camp bed, a kettle, a rail with her three suits on hangers, and a window the width of a book that showed her the ankles of passers-by. She had lived in it for a year because the rent was nothing and it was the only place in the building without a smart puck. She lay with her arm throbbing and the blanket pulled to her chin and counted. Ceiling tiles: twenty-four. Exits: one, a door with a stuck bolt.

Then the voice from the other side of that door said, "I'm not going to say 'rise and shine' because you look like a ghost, so instead I'm going to say there is soup."

"Seun."

"Soup, a croissant, and a pastor's amount of guilt." He kicked the door open with his foot and came in bearing a tray, in his work lanyard, dressing gown abandoned, hair standing up in a style he called Mild Shock. "You've been asleep six hours. Rui carried your phone down, he said you'd want it, and then he stood in the corridor for ten minutes not knocking. It was the most romantic thing I've ever seen. Eat."

"It's not romantic. It's evidence."

"All romances are evidence, my love." He set the tray on her knees. "The landlord's agent is coming at five to look at the ceiling, and Hearthwell are sending a woman to check the smart-home system after the incident, which is a thing they're now calling the incident, and I've told the residents it was corrosion, and I've told the insurer it was an act of God, and I've told God it was corrosion, so, frankly, I'm exhausted."

"Seun."

"I know."

"The clip—"

"I know, Zee." He said it gently, and the gentleness was a weight. He sat on the end of her bed and looked at his hands. "I didn't watch it. I want to say that. I've never watched it. I'll never watch it. But I saw how you moved last night and I'm not stupid. I'm just good at being stupid in front of people. You can tell me or you can not tell me and I'll bring soup either way."

She held the spoon. Soup was a miracle. She said, because it was true and it was the only honest thing she had left, "I'll tell you. I need a day."

"Take two."

"I need to know what it is first."

"Then know it," said Seun, and stood, and stopped at the door. "But Zee? Pip's outside. Six-C Pip. She's been sitting on the stairs since noon, doing that thing teenagers do where they look at their phone and then look at you like you owe them rent. She wants to talk, and she's got her mother's face on."

"What does that mean?"

"It means somebody's going to be told off."

Pilar Nyambura-Voss was seventeen years old and had four hundred thousand followers who knew her as pipcuts, a girl who took any video you gave her and made it better and meaner. She sat on the bottom step of the service stairs with a laptop on her knees, wearing a hoodie two sizes too big and black eyeliner which she had applied with a ruler and a vendetta. The moment Zee appeared she snapped the lid shut.

"Right. First. I'm not going to say sorry for filming. It was news. The ceiling fell on a desk. Second. You're a total liar."

"I am."

"Oh. Okay." It threw her for two seconds. "Good. That helps. Third. I watched your clip."

Zee sat on the step above her. It was cold. The stairwell smelled of old wax and somebody's frying. "Which clip?"

"The Zee Blinked one. The one in the chat. The long one." Pip looked at the wall. "Gus sent it to me because he's an idiot and I watched it in our kitchen, because that's where I do my best thinking, because it's got the good light. And it was longer. It went on. It had our kitchen in it, Zee. From the top of the fridge."

Something went through Zee like a cold wire. "What happened in it?"

"My mum slipped." Pip's voice did something odd and then recovered. "I was standing at the sink doing the thing you do when you're pretending to do chores, and she came in from her shift in her coat, and there was oil on the floor from where I'd made chips, I always make a mess, and in the clip she goes down. Hard. She hits the corner of the table with her head. And then the picture just stays there. On her. For the last, like, I don't know. It felt like an hour."

"Eleven seconds."

"Yeah. You know it's eleven? How do you know it's eleven?" She stared. "Oh my God. You know everything."

"I know some things. What did you do?"

"I moved the oil. I put a tea towel down. Mum came in and said, what are you doing, you strange child, and I said wiping, and I stood in front of her for the full eleven seconds like a lunatic, in case, and she didn't fall. And then I played it again, because obviously I played it again, and it showed me standing in front of her like a lunatic. Like it had been waiting to see what I did." She finally looked straight at Zee, black-rimmed, enormous, frightened out of her pride. "She'd have really hit her head. It would have been the corner. Zee, I'm not being dramatic."

"You're not."

"I'm usually dramatic."

"I noticed."

For one second, one precious second, they nearly laughed. Then Pip opened the laptop again, and her face closed.

"So I did what I do. I uploaded it. Just the new bit. I thought, it's the biggest thing I've ever had in my hands, this is the real thing, a clip that sees the future, I cut a thirty-second reveal, and it went up on Loop at twenty past one. Zee, it has nothing in it."

"Nothing?"

"Thirty-four seconds. The terrace and the knife. Then it just stops. That's all anyone else sees. I've put it on five different accounts. Same. Just the old clip. People are saying I've faked a reaction to your meme." She hit a key. On the screen the thread of comments scrolled. DO A ZEE, one said. LMAO SHE BLINKED AGAIN, another. "It only works in here. In the chat. It's like it only wants us."

"Who's us?"

"I don't know. The building?" Pip shrugged violently. "It's the only place where it's long."

Zee took out her own phone. She hadn't looked at it since the morning. The Loop thread was still there, in the residents' chat, below Gus's message and the seven hundred replies of people saying thank you for being awake. A video, a thumbnail. She did not play it. She looked at the number under it.

The clip had been 1:29 when she'd left Rui's flat at nine.

It said 2:07.

"That's four more plays," said Zee.

"Yeah."

"Not mine. I haven't touched it."

Pip's face went through three expressions in a row and arrived at the one that said she had been on a different thought for much longer. "I played it twice. In the kitchen. Gus has played it, he said he wanted to see what the fuss was. Your friend with the headphones must have. That's four. But I looked at the new bits, because I'm a freak, and, Zee." She tipped the laptop. "They aren't all in our kitchen."

The new footage was lined up in a strip under the video, the way her editing program liked to show it. Eleven seconds, eleven seconds, eleven seconds. A fat strip of thumbnails. The first, the lobby. The second, Rui's flat. The third, a kitchen with orange cupboards. The fourth, which she had not seen, was a yellow bathroom, a stranger's feet in slippers in a tub.

"Gus's," said Pip. "He plays it in the bath. Of course he does."

"And the last?"

Pip clicked on it. "I didn't play this one. It's not mine and it's not Gus's. It came at 2:07. I think someone was playing it just before you woke up. It's still, it's still going."

On the laptop screen, a corridor. A carpet the colour of old pavement. A row of doors with brass numbers: 4A, 4B, 4C. The picture was held low, not from the ceiling this time but at about the height of a person's knee, and there was a light behind it that flickered like a phone screen. The picture trembled. Then, far down the passage, the door of 4B moved, and a thin shadow slid along the floor towards it.

A hand came into frame from the left. A pale hand, with a square gold ring, the sort you wear when you inherited something. It took hold of the brass handle of 4B, the one with the cheese behind it and the machines and the man who fixed old tape.

It turned the handle, gently, to see if the door was open.

Zee was on her feet. She could not remember standing. Behind her, Pip said, "What? What is it? Zee, it's still playing, it's at fourteen —"

"Is it happening now?"

"It's — I think — the counter's still —"

"Pip," Zee said, already on the stairs, taking them two at a time. "Stay here. Do not play it again."

She went up four flights with her bad arm held against her ribs and the counter going in her chest. One flight. Two. She did not know the owner of the hand. She knew the weight of a person going into a room. She had spent years learning the weight of a person going into a room.

On the third landing the lights went out.

Not all of them. Just the stair lights, and the corridor light on the fourth floor above, in a single soft click, as though someone in a control room had found the right switch. In the sudden dark she could hear her own boots. Beyond them, up in the well of the staircase, very faintly, there was another sound. A voice. A small, bright, domestic voice that she had heard in every flat in the building, calling out weather forecasts and playlist suggestions to people who weren't listening.

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