When high school senior Charlotte agrees to babysit the Wilbanks twins, she plans to put the six-year-olds to bed early and spend a quiet night studying: the SATs are tomorrow, and checking the Native American/Alaskan Native box on all the forms won’t help if she chokes on test day. But tomorrow is also Halloween, and the twins are eager to show off their costumes.

Charlotte’s last babysitting gig almost ended in tragedy when her young charge sleepwalked unnoticed into the middle of the street, only to be found unharmed by Charlotte’s mother. Charlotte vows to be extra careful this time. But the house is filled with mysterious noises and secrets that only the twins understand, echoes of horrors that Charlotte gradually realizes took place in the house eleven years ago. Soon Charlotte has to admit that every babysitter’s worse nightmare has come true: they’re not alone in the house.