The door of 1A had a brass knocker in the shape of a hand holding a ball, and Zee had never once seen anyone use it.

She used it. Three knocks, evenly spaced, then a fourth. She heard herself do it and was irritated to find she had caught the habit off Rui, like a cold.

She had left them upstairs. Seun was on the sofa in 4B with a damp flannel over his face, saying "I'm a monster" at intervals in a muffled voice, and Rui had been making him tea in the tone of a man defusing a bomb. Zee had told them she needed air. She had gone down four flights, past the stairs that lit themselves under her feet, to the one door on the ground floor that was not the lobby, and stood in front of it with her gloved hand hidden in her pocket.

"It is open, child," said a voice from inside. "It is always open for people who knock. Whether they enter is a different matter."

She entered.

The flat smelled of floor polish and boiled coffee and a faint sweet note under it, like an old library, like warm dust. It was one long room divided by shelves, and the shelves were full of reel boxes, labelled in a small cramped hand, each box grey with the soft fur of long years. A tall window looked out onto the courtyard. In an armchair beside it, wrapped to the chin in a mustard shawl, sat Dona Graça Mbeki-Sequeira with a cat on her knee and a cane across the arm of the chair, like a sceptre.

The cat, Fado, a long grey animal with a bent tail, opened one yellow eye and shut it again.

"You are late," said Dona Graça. "I expected you two hours ago. I heard the bell of the visitors from here. Sit."

Zee did not sit. "You know what she said to me."

"The pipes carry. A woman who keeps a building for forty years does not need a microphone." The old woman's eyes, black and bright as a bird's, went over her without hurry. "The recording from Marseille. A longer version. Yes. I know. And I know you want it, and I know you are ashamed of wanting it, and I know that if you play it in that woman's company, you will not come back from the part she wants you to hear. So. Sit."

"I have been told to sit twice today."

"By whom?"

"A speaker."

"Then do the opposite, engineer. Stand if you must. But you will not stand long."

Zee stayed on her feet, out of stubbornness, for another eleven seconds. She counted them. Then she sat on the edge of a hard wooden chair that was clearly kept for visitors who needed to be put in their place, and the cat, in a single contemptuous movement, turned its back.

"Dona Graça," she said. "The cylinder. Z. Oyelaran, 1911, not played. You told me it was the reason the clip found me. You told me nothing else. I've since met my great-grandfather. In my lobby. He's in a queue. There's a woman at the back of it who has my face. I have been told my name was spoken by someone else's mouth, and I've been enrolled in a research programme by a man who cries into a flannel, and the company that did it has a recording of the worst night of my life and wants to play it to me like a favour. I need the rest."

"You need a great deal, child. It is a modern complaint."

"Please."

The old woman said nothing for a while. She stroked the cat. Outside in the courtyard a blackbird was making a sound like a rusty gate.

"You have said please twice since you came in," she said at last. "I have been listening to you for many years through the floor of the lobby, and I do not think I have heard you say it once. So I will break a rule." She shifted. "Fetch me the second drawer."

The cabinet was a tall one of dark wood, with brass pulls the size of walnuts. The second drawer slid out heavily on its runners. Inside, in tidy rows, stood narrow cards in cream-coloured stock, hundreds of them, with a faint brown fringe along the edge where damp had got at them. A card index. The kind of thing the whole world used to be kept in.

"My mother's," said Dona Graça. "She typed it in 1955, from the labels on the cylinders, because the Office had no catalogue and the Director was a vain man who wished to be thought organised. It took her four years. She typed it in the annex, in the room under your reception desk, with a mask on her face because of the mould. Find the O's."

Zee's fingers were not quite steady. She walked them across the tops of the cards. Oduya. Ogunbayo. Okafor. Olusegun. Onyeama. Oyelaran, Segun. She stopped. She drew it out.

It was a small, frail, yellowing card, typed in a worn blue ribbon. OYELARAN, SEGUN. Carpenter, Docks. b. c. 1881, Lagos. Rec. 14 Mar 1911. Cyl. 0388. STATUS: NOT PLAYED. REFUSED CLAUSE 3. There was a small circle in pencil beside the last word, and next to it, in a hand that was not the typist's, a scrawled exclamation mark.

She was holding her great-grandfather. It was one inch by four inches, and weighed nothing.

"Behind it," said Dona Graça.

She turned the card over, and found a second one stuck to it with a bit of brown paper tape. A newer card, much newer, the stock still faintly white. The typeface was different: a bold, tidy face, a different machine.

OYELARAN-PEREIRA, Z.

There was a date after it. 1991.

She read it three times. The words lay on the card exactly as they lay on every form she had ever filled in. The z, the hyphen, the comma. Under it, three lines.

Expected. Not yet received.

Held in trust under Cyl. 0388.

Beyond the last line, in pencil, in the same pencilling hand as the exclamation mark, someone had written: She will blink.

The room did not go dark. It simply became the sort of silent a house makes when everyone in it has stopped breathing.

"I was born in 1991," Zee said. Her voice came out remote, reasonable, as if she were reading out an instruction for a fire extinguisher. "My mother named me in the delivery room. My dad wanted Ifeoma. Nobody knew the hyphen until the registry."

"My mother typed that card in 1959," said Dona Graça. "She did not know what it meant. She typed what she was given, from a slip that was left on her desk one morning with no name on it, in a hand not the Director's. She was a faithful typist. She typed it, and filed it, and for the rest of her life she did not mention it, and on the night she died she said to me, in the voice of someone repeating a thing learned very young, when she is called, she will blink. Child, I thought it was delirium."

"But you kept the card."

"I kept everything. It is the only talent I have."

Zee put the cards down on her knee. They were, absurdly, trembling. She became aware that her left hand had come out of her pocket, and the glove had gone slack, and in the window light the bleached hand lay in her lap, pale as a plate, and a little translucent at the heel. The old woman's eyes went to it. They did not flinch.

"It was a prophecy," Zee said. "You're telling me someone wrote a prophecy."

"It was an index." The old voice was dry as paper. "That is the thing you must understand, engineer, since you are so fond of precision. The Office did not foretell. It filed. It had a system. Everything that came into the world was meant to arrive in the right drawer, in the right order, and the Office was a hungry thing, and a very orderly thing, and it made a place for you before you were born, because it was sure you were coming. You did not walk into its net. It was strung for you."

"Why me?"

"Because your great-grandfather would not say clause three, and an unfinished record is the one thing it cannot bear." She tipped her head. "It has been waiting a hundred and thirteen years for someone of his blood to finish the sentence."

The radiator ticked. Fado stretched on the old woman's knee and began, with great deliberation, to wash one paw.

"What is clause three?" Zee said.

"Ah." Dona Graça smiled a thin, hard smile. "That I will not tell you. Not yet. A person cannot be asked to withhold what she does not know. If you know the line, you will say it. Everyone says it, in the end. I have not decided whether you are everyone."

"That's a test."

"All my life is a test, child. It is what I do instead of knitting."

Zee laughed. It surprised both of them. It came out cracked and wet, and she put the heel of her good hand to her eyes and pressed until she saw colours. When she took it away, the old woman was holding out something small across the arm of the chair.

A key. An old Yale on a loop of green wool, worn silver at the teeth.

"The front door of this flat," said Dona Graça. "I have not given it to a living soul since 1983. There is a pantry behind the shelf with the reel boxes, and in the back of it a door, and behind the door a flight of steps. I will not tell you what is at the bottom. I will tell you that it is dry, and that the voices are not in the building's walls, as the boy believes, but in the cellar, and that I would rather you went down it with my key in your hand than with that woman's permission."

"You said you wouldn't talk about the cellar."

"I am not talking. I am giving you a key." The black eyes held hers. "You will go when you are ready. Not tonight. Tonight, you will sleep, or you will do whatever it is that you do instead, and you will not play that clip, and you will not let that woman play anything to you. And if the pale one, the one at the back of the line, comes to the desk, you will not turn your head to see her. You will count your breath, and you will hold the lobby. Do you understand me?"

"How do you know she's close to the desk?"

"Because I have been counting too." Dona Graça adjusted her shawl. "Seven prints between the door and the desk, in the old plan. The Office is orderly. Seven places. You have used six uses and your double has taken six of them. One place remains."

Zee closed her hand around the key. It was warm from the old woman's lap.

"One more use," she said.

"One more."

"And then?"

The cat stopped washing its paw. In the courtyard the blackbird fell silent, as if it too had been listening, and into the quiet, from somewhere far beneath the floor, there rose a small, dry, patient sound, the scratch of a pen on a long ruled page.