What I find is my son, sitting cross-legged on the floor at the end of the hallway in his school uniform, looking for all the world like he’s waiting for assembly to start.

‘What the hell is going on?’ I demand, relief making my tone sharper than I intend. ‘Are you all right? Where’s your mother?’

‘Stacey and Harper had a fight,’ he says.

‘What sort of fight? Where are they now?’ I glance up the stairs to the kitchen, and then back out into the street. ‘And where on earth is your mother?’

‘Stacey locked us in the cellar,’ he says. ‘She put me in the freezer. Mummy got me out, and Harper came to rescue us, but then Stacey caught us and she and Harper started fighting.’ He delivers all this in a strange monotone, as if he’s rehearsing lines from a play. ‘Harper told me to call you. But I couldn’t find my phone and then there was an accident and I didn’t know what to do.’

The only thing I can get my head around in this insane catalogue of horror movie clichés is that there has been an accident and I still don’t know where my wife is.

‘Where are they?’ I shout. ‘Peter, where are they?’

He points dumbly towards the other end of the hall. A half-hidden flight of stairs leads down into the dark.

I run towards the stairwell, shouting Millie’s name. A faint sound comes from the cellar: a thump, then a low groan. A wash of panic floods through me as I pull out my phone for light and take the stairs as fast as I dare, terrified of what I might find at the bottom. If that woman has hurt Millie, if anything has happened to my wife, I will kill her, I swear to God.

I almost trip at the foot of the stairs. My foot connects with something warm, soft: someone.

‘Oh, Jesus, no,’ I breathe.

But it’s not Millie crumpled at the bottom of the concrete steps.

I crouch down beside Harper. She has a deep gash on her forehead, and it looks like her leg is broken: I’m sure her knee isn’t supposed to hinge like that. I feel queasy: I’m not good with blood. I think – oh, Christ – I think I can see bone poking through her skin.

‘Don’t try to move,’ I say, swallowing hard. ‘It’s going to be OK, Harper. I’m going to get help.’

‘Millie,’ she whispers.

I catch a glimpse of tangled limbs in the shadows just beyond Harper, and swing my phone upwards in a panicked arc.

Stacey must have broken Harper’s fall. She’s sprawled on the cellar floor, her neck twisted at an unnatural angle, her eyes wide and sightless. I’m not a doctor, but even I can see at a glance she’s dead.