Cassie's night of babysitting goes to hell when bombs explode in the distance, planes fly overhead at low altitudes, and alerts on her phone tell her to seek shelter. Luckily, the boy she babysits tells her his father has a bunker in the yard.

When they make their way to this underground shelter, they soon discover they aren't alone. Something is living in the bunker, lurking in the walls, and it's hungry.

What started as a simple night of babysitting will descend into a psychological and claustrophobic nightmare Cassie couldn't have prepared for.