There's nothing too scary about a mean babysitter - Dervla bosses and bullies, makes the kids eat their vegetables and never lets anyone stay up past bedtime. But one little girl decides to rebel: she writes on the walls. She makes prank calls to China. She puts spiders in Dervla's hair. At first she's just having fun, but when she sees two black horns poking out of Dervla's frizzy red mop, she realises what she's dealing with: pure evil.