I told her everything, just as she’d asked. As I talked, I was surprised to hear hollowness in my voice, but Olivia made interested noises. Occasionally she gasped or groaned or rubbed my shoulders, while I twisted my hands together in the darkness, glad to be invisible as I tried to slow the storming of my heart. Talking about the past ten years was like inflicting lashes on my own back.

Why did she have to choose me, choose anyone?

Because she’s insane.

There was, I told myself, a reason for her madness. There had to be. People weren’t born evil – were they?

I sighed. ‘I think her brain’s all muddled up. I think, sometimes, she believes she really did save me,’ I said quietly. ‘She wasn’t horrible all of the time. Sometimes, when I was little, she played with me, baked cakes with me, read to me at bedtime …’

‘You’re being too generous,’ Olivia said.

‘What do you mean?’

‘You’re trying to make excuses for her. Trying to understand her. She told you that you were allergic to light, for Christ’s sakes. She stopped you going outside. And now she’s locked you in a hole in the ground.’

‘I know …’

‘She’s stone-cold crazy, believe me. I know – she’s always been crazy.’

I said nothing. I didn’t want to force Olivia to tell me anything.

Silence flooded the space between us, and my eyes grew heavier and heavier. The darkness pressed down and an irresistible sleepiness teased my mind. It was silent for a long time. I began to drift away.

Olivia sighed heavily. ‘Four. We were four when I started noticing something strange about Catherine.’

I jerked upright, eyes wide, suddenly alert. I wanted to know everything. I wanted to understand her. Mother. Understand how she could bring herself to do the terrible things she had done.

‘Strange?’ Catherine. So that’s her name.

‘Yes. Strange. Disturbing. My earliest memory is her twisting the head off a doll then stamping on its face. I think she was angry because she’d been told off. I don’t remember the details. She always had a terrible temper.