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.

Mara reaches the door and looks in. The crib is empty, the blanket thrown back. Then she hears humming behind her and turns. Theo stands at the top of the stairs in his pajamas. 'There's another me in there,' he says, pointing past her. 'He wants to trade, and he's stronger than me.' Mara grabs the real Theo and runs out into the night, and never looks back.