There must have been a reason Stacey turned on Peter. A reason she tried to kill him. Something he’d done that she found unforgivable.

He thought he could do what he liked and it’d be too late for me to be able to do anything about it.

There’s a story in Native American culture that describes the battle between two wolves that live inside us all: one wolf is evil – it is anger, violence, resentment, darkness, despair. The other wolf is good – it’s love, hope, light. Which wolf wins?

The one you feed.

It’s not too late for Peter. I can still save him, if Tom will let me.

‘How could he?’ Tom whispers. ‘How could he be part of this … this horror show?’

‘He’s our son,’ I say. ‘He’s ours.’

Tom puts his hand over his mouth, as if physically holding back words.

‘He’s our son,’ I insist. ‘He’s my son.’

‘This isn’t you,’ Tom says harshly. ‘Don’t you dare put this on yourself. He didn’t get this from you. You’re nothing like this. You’re not a monster. You save lives, Millie, you don’t take them. You save lives,’ he says again.

He’s right. The dark wolf in me is strong, but I’ve worked hard all my life to starve him into submission. My childhood forced me to armour myself against the world, to detach myself emotionally for my own self-preservation. But I’m not a psychopath: if I were, I wouldn’t distinguish between the good wolf and the bad. I wouldn’t even know they were different.

I’d be like Peter.

My son’s forced me to confront what true psychopathy looks like. But he can’t help the way he’s wired any more than I could help the family I was born into. I have to save him if I can.

‘We can’t tell anyone he knew,’ I plead. ‘We’ve got to protect him.’

‘Who’ll protect everyone from him?’

‘Tom,’ I beg. ‘Please. We don’t know. We don’t know he did anything wrong. We don’t know.’

Harper moans softly on the cellar floor beside us. In the distance, the sound of sirens grows louder. The ambulance is nearly here. ‘Hold on,’ I tell her urgently. ‘Think of Kyle and the boys. You can do this, Harper. Hold on for them.’

Her breathing is getting shallow. I’m a doctor: I know the signs.

She’s twenty-seven years old. She has two little boys called Tyler and Lucas.

She saved my son’s life.

I stroke her hair back from her forehead. ‘It’s going to be OK,’ I lie. ‘You’re going to be fine. You just have to hold on a little bit longer.’

‘Peter,’ she whispers.

‘He’s OK. You got him out. You saved his life, Harper.’

‘Peter,’ she says again.

I lean in close.

‘Peter … pushed … us.’