The Brubaker house always smells like other people's dinners. Mara has babysat three-year-old Theo eleven times now. Same rules every time: monitor on, door cracked, bed by eight. It's 9:40 and the living room is dark except for the little green screen. Theo is a small white smudge, asleep on his side. Then the screen hisses with static, and when it clears, the smudge is sitting straight up, facing the camera.

Mara stares at the screen. Theo never sits up like that, stiff and still. She turns the volume up. Through the speaker comes a soft sound. Not crying. It's humming. A slow, tuneless hum she has never heard him make. She gets up to go check on him.

Halfway up the stairs, the humming on the monitor stops. Then a new sound takes its place: footsteps. Small bare feet on hardwood, moving fast. But Theo's door is still shut, and the steps are coming from behind her, down in the dark hall she just left.

Mara whips around. The hall is empty, but the bathroom door at the end now hangs wide open, light off. The footsteps came from there. On the monitor in her hand, the crib view shows Theo asleep and still. So if Theo is in bed, what just ran across the hall?