She despises that sound, the grating of enamel against itself. She touches his jaw, trying to hold it still. Edward only grinds his teeth when something is on his mind. He’s been doing it a lot lately.

But she’s hardly seen him this week, and when their lives have collided over a late dinner or half an hour’s television, she hasn’t liked to ask why he’s been keeping her at arm’s length.

She knows it isn’t a woman, at least not in that way; even though the years have deepened his boyish good looks into the kind of crumpled attractiveness that makes the school mothers look twice, Edward’s always had strident views on fidelity. But he is short-tempered and secretive with his phone, shutting down the screen whenever she surprises him, staring at her with an expression she cannot place.

Last Thursday night, he lied to her. I’ve been playing squash, love. With Mark from work. But one of the fathers from football saw him getting off the train from London when he was supposed to be on the court.

On her bedside table, the clock clicks forward another minute. Each tiny hair on her arm rises until her skin is stippled with goosebumps.

4.07 a.m.

Even now, that combination of digits has the power to root her in place.

Twenty-one years ago, in another bedroom and another life, a different clock stopped forever at precisely the same time, a web of hairline cracks across its face.

Catherine never speaks about that night. The past is put away now. It will not define her. She tries her best not to think about it, but every now and then it shoves its way in, ugly and unwelcome. The devil’s clock, her mother called it, those hollow hours before dawn when the darkness is full of horrors and sleep will not come.

Her memories unfold like a macabre chain of paper dolls.

The bodies of her parents. A black pool spreading across the carpet. The moon’s reflection in the handle of the scissor blade buried in her father’s neck. Her mother’s limp hand hanging over the bed, pastel-pink varnish on her nails.

Her older sister, pyjamas soaked with blood, blonde hair like a halo, being led away by police.

How she had loved her then.

Catherine closes her eyes and calls up Honor’s face, all braces and apple-red cheeks. Laughing and dancing, making up songs. Warm hands. Wholesome and happy. A talisman to protect them all.

A car door slams outside.

Edward stirs again. Catherine half rises from their bed. In this pretty village on the anonymous edges of an Essex market town, most inhabitants are asleep by ten o’clock.

As she crosses the bedroom to the window, arms wrapped around herself to keep out the cold, a blue light begins to pulse against her floral wallpaper, a neon wash that keeps time with her heartbeat.