In this thriller (which represented something of a departure for Hammer Films, noted for their gothic period pieces), Joey Fane (William Dix) has returned home after two years in an institution for mentally ill children. His sister drowned, and his family believes that Joey was to blame, despite his claims of innocence.
Joey is convinced that the family's Nanny (Bette Davis) was responsible and refuses to have anything to do with her, but only neighbor girl Bobby (Pamela Franklin) agrees that there's something sinister about the woman minding the house.
When Joey's neurotic mother Virginia (Wendy Craig) nearly dies after eating tainted food prepared by the Nanny, Virginia's sister Penelope (Jill Bennett) comes by to help. Penelope soon witnesses the bad blood between Joey and the Nanny, though before long she begins to think that the boy might be right about her after all.
Jimmy Sangster adapted the screenplay from a novel by Evelyn Piper.