Did he know what Stacey was planning to do when he slipped silently out of the house that night? Was she waiting for him outside in her stolen car? I find it hard to imagine she’d want a ten-year-old boy along for the ride, but the two of them have been drawn to each other from the day they met. It sounds ridiculous: a child and a grown woman, a successful woman, a mother herself. But there’s a darkness between them, a kinship, that makes me fearful. Harper says the intense bond between them creeps me out and I can’t say I disagree with her. Together, they’re far more dangerous than either of them would be alone.

I think of murderous pairings like Myra Hindley and Ian Brady. They brought out the psychopathic worst in each other, committing vicious, sadistic crimes together that were almost inconceivable in their brutality. Would they have tortured and murdered children had they been acting alone? They recognised something of themselves in each other, something misshapen and ugly and deformed, and drew strength from that black affinity.

Age isn’t a barrier to this sort of evil partnership: the killers of toddler James Bulger, abducted from a shopping centre before being tortured to death and abandoned on a railway line near Liverpool, were just ten years old themselves at the time.

Exactly the same age my son is now.

I cast my mind back to the morning five weeks ago when Stacey came to our house with a black eye, the same day Tom asked her to babysit Peter. It was only a few days after my son had tried to drown hers in the swimming pool at the Hurlingham Club. I remember every instinct warning me of danger! but my fear was for Stacey, not Peter. It never occurred to me for a split second that the threat was not to either of them, but from both of them.

Two days later, Felix Porter disappeared.

If my son was partly responsible, if he’s kept this dreadful secret for five weeks – and I know better than most people the secrets children are capable of keeping from their parents: they know all of ours, but we know none of theirs – then I have to help him, because sooner or later Stacey will tire of her game of cat and mouse, and then his life won’t be worth spit. Because despite her denials, I’m quite certain Stacey has killed Felix. And now I’m equally sure Peter helped her.

Tom knows this too, but he won’t admit it to himself, much less to me. He insists Stacey’s lying: that it can’t have been Peter in the car with her. Which means he doesn’t have to choose between saving our son or saving the world from him: he abdicates that bitter responsibility to me.