‘There’s something I have to tell you,’ muttered Deirdre. ‘Something you have to know. She’s torturing us, but she’s telling the truth.’

Her ghost-child grandmother had disappeared up ahead of them somewhere, but Deirdre felt listened to, spied upon, constrained. And – disloyal.

‘I could tell you her story. I know it backwards. I’ve heard it over and over, again and again, the same terrible tales, until I thought I would go mad from hearing them.

‘But none of the details really matter. Only one thing matters, Gal: that poor little frightened girl – that poor little frightened girl inside my grandmother. Do you see what I mean?’

Gal’s face was stony. That poor little frightened girl was the monster he wanted to free Deirdre from. His anger was too deep and too well justified to forgive her so easily. Deirdre gazed into his eyes. Then she sighed.

‘My grandmother,’ she began, ‘was born in Corbenic, when Corbenic was still a house, before her father built the rest of the flats around it. And before she started adding to it. Corbenic was built in stages, you know. That year, 1936, on the glass door in the entrance, was when my great-grandfather finally completed them. His version of them. But they’d been a good business for years before that. So my grandmother was born in the oldest part of this building. But her mother, my great-grandmother, died when she was born. My grandmother’s own grandmother looked after her when she was a baby. But then her grandmother died too. And then Grandmother’s father – my great-grandfather – met Elaine.’

Gal placed his palms across his face.

‘I know. I know. I know,’ he said from behind them. He said it wearily, but not without sympathy. ‘And she felt abandoned. And it was horrible. And she never got over it.’

But Deirdre wasn’t listening.

‘Elaine was old by the time we met her, of course,’ she said. ‘It had all happened so long ago! But I wasn’t allowed to know her. Grandmother told her to keep away from us. This is a big building – it’s quite possible for someone to live here and never meet anyone they don’t want to meet. Elaine had been living here since my great-grandfather died, and that was years before I was born. My great-grandfather had left her a flat, and she had just stayed on.

‘Grandmother hated Elaine. From the day she first met her, until the end of her life. When Great-grandfather took up with Elaine he neglected my grandmother – travelled the world with Elaine – that was Corbenic’s heyday, and there was money to burn – and, yes, my grandmother felt abandoned. She was abandoned. She never forgave my great-grandfather. She loved him so much, you see. And she never forgave Elaine, because she stole him away. That’s really how it all started. Her unhappiness, I mean. And her hatred of men.

‘And she did hate men, you know. They had hurt her so badly.

‘When she was sixteen she ran away to get married – she was desperate to get out of this place, with her father and his mistress making the rules. But she wasn’t happy. And it didn’t last. She had a little boy, a beautiful little boy, who died. She never got over that, either. It was horrible. And it was another reason to hate Elaine, because Elaine had a little boy too – my grandmother’s half-brother. And he lived.’

Deirdre paused and gazed searchingly at Gal. Then her thoughts seemed to drift back to the story of her grandmother’s life.

‘Then, years later, she had my mother,’ she said sadly. ‘And then her husband left her. And when my great-grandfather died she moved back here. And changed her name back to Dark. She adored my mother. But when my mother got pregnant with me, my grandmother was so disappointed that she never forgave her. My mother was just a teenager, you see, and my grandmother had had such high hopes for her. I knew all that.

‘But my grandmother always told me I was abandoned, that my mother had just run away and left me, and that she had never heard from her again. I never knew my grandmother sent my mother to boarding school, and took me away from her, and told her never to come back . . .’

‘But you’re not angry with her? Why aren’t you angry with her?’

‘Oh, Gal, you don’t understand. You think she’s a bully, and she is. Was. But there was a reason she bullied us. It was because she was afraid. I was all she had. I was so precious to her. I’ve never understood before just how desperate she was to keep me for herself.’ And again, suddenly, she searched Gal’s face with her eyes. ‘How do you leave someone who needs you so badly, Gal? How do you do it?’

And Gal could find nothing to say in reply.

‘After Grandmother banished you, all through those years before I started at the high school, she got stranger and stranger. She kept telling me the story of her life, over and over again. And she couldn’t stop building. The whole town was talking about it. The business was failing – she had fewer and fewer tenants, and she had to make the rents lower and lower, because she would never spend any money on the upkeep of the building. There were so many things that needed fixing, but she wouldn’t fix them. Instead she used the money her father left her to build more and more extensions – stairs and hallways and landings and vestibules – and yet none of them had any function. She made Corbenic into a labyrinth. It must be twice the size it was when you lived here. The yard is gone now. The building covers it.’

‘But why?’ said Gal. ‘Why did she keep on building?’

Deirdre stared at him.

‘It was as if she was trying to protect herself from something, get away from something, put distance between herself and something. It was as if she was burying something beneath more and more layers of stairways and halls and landings. It was as if she was building a fortress. It was as if she was running from something and the extensions were the path of her flight.

‘Gal, there is something in the building, something my grandmother is terrified of. And I think it must be the thing we found when we were five. The most important thing. The thing we can’t remember. The thing you were banished over. The thing we’re looking for now. Even now, she wants to destroy it. She wants it to be lost forever.’

Gal stared at her, digesting what she’d said. He was beginning to understand, better than he’d ever understood before. But . . .

There was something nagging at him, something important that he couldn’t quite grasp. And yet he couldn’t put his finger on it.

Then suddenly he said, ‘So what happened to Elaine’s baby, the little boy your grandmother was jealous of?’

‘Oh, Gal,’ said Deirdre sadly. ‘That’s what I needed to tell you. Haven’t you guessed yet? He was your father. You are Lainey’s grandson. That’s why my grandmother hated you so much.’

For a moment Gal thought he might be going to faint. For a moment the whole world turned upside down.

He put a hand out to steady himself against the wall, and remembered, as he did so, the bizarre fact that the building was in the process of collapsing around him.

He had always known that he was related to Deirdre; he had always known that he was related to her grandmother – his surname, like theirs, was Dark. But no one had ever told him how. If he had thought about it at all, he had imagined the relationship to be a distant one.

Now he realised that Deirdre and he were descended from the same man; that her great-grandfather was his father’s father.

Now he knew that, in old-fashioned terms, his father was illegitimate.

And now, for the first time, he understood something Deirdre’s grandmother had said to him, long ago, during that argument, when she had said his family was not trustworthy, and he had argued that she and he were from the same one.

‘Yes, but you’re from the funny side of it, aren’t you? The wrong side of the sheets. Little bastard . . .’

Suddenly a hundred things made sense; a hundred small mysteries were solved. For if Elaine was his grandmother, he saw how, in Deirdre’s grandmother’s eyes, he had inherited her guilt. Even worse, he saw how he had inherited her father’s guilt, for in a way he was the product of it – the descendant of it. He was the male she could punish for her father’s sins, in her father’s stead. And if that wasn’t enough, there was the rivalry over Deirdre, the most precious thing in her life. He was trying to steal Deirdre away from her. He was her natural enemy.

But then he remembered something else the ghost child had said. He’s the reason I took my revenge on you.

What did she mean by her revenge?

What was it she had done?

He felt a coldness near him, as if someone had opened a window onto an icy winter’s night. He glanced up. And when he did, he saw something that frightened him so much he felt for a moment as if he was losing his mind.

It was her. It was the ghost-child grandmother. She was standing in front of him. But her feet were so far from the ground their faces were level, and when he looked in her eyes he saw death, death, death.

Her face was a kaleidoscope again, one moment a child’s, one moment an old woman’s, one moment that of a corpse in the grave. She had crept back while they were talking. She had been listening quietly to what they were saying, although she looked puzzled, as if, like a five year old, she couldn’t completely follow the drift of their conversation.

‘You didn’t come,’ she said, like a little girl who had been waiting too long to be found in a game of hide-and-seek. ‘Why didn’t you come? I’ve got something else to show you. The best thing of all.’