Virginia Fane is frightened. Her ten-year-old son, Joey, is due home after a two-year period in a special home for mentally disturbed children. Joey was admitted to the home after the tragic death of his young sister, who was found drowned in her bath in suspicious circumstances.

Before leaving the special home, Joey plays a wicked joke on the matron, pretending to hang himself in his room. The psychiatrist reveals to Joey's father that the boy has an innate antipathy to middle-aged women. This is confirmed when Joey realises that his Nanny has accompanied his father: he is immediately rude and hostile to her. On his return, although he is pleased to see his mother, he dislikes his room, which Nanny has prepared for him, and refuses to eat because Nanny has cooked it. Later, when he discovers there is no key to the bathroom door, he makes Nanny swear in front of his mother that she will not enter the room while he is taking a bath. Joey's behaviour angers his parents, to whom Nanny is indispensable; she has been with the family since Virginia was a child.

When his father, a Queen's messenger, is called away on official business, Joey becomes even more antagonistic to Nanny, accusing her of poisoning his food (an accusation which previously had contributed to his being sent away to the special school). Witnessed by the neighbouring doctor's young daughter, Bobby, who he has recently befriended, Joey contrives to play a cruel trick on Nanny. He leaves a doll face-down in the bathwater for her to discover when she pulls back the shower curtain, reproducing the circumstances in which she discovered his sister's body. The game has the desired effect: Nanny is deeply shocked. His mother is appalled by Joey's malicious and insensitive behaviour. They have a violent argument, in which Joey says he hates her.

Later that night his mother is rushed to hospital with suspected food-poisoning, for which Joey is blamed. In her absence, her sister, Penny comes reluctantly to baby-sit. Penny's weak heart (resulting from a childhood bout of rheumatic fever) is sorely strained when she is woken up by Joey, who claims that Nanny has tried to drown him in the bath; Penny strikes him.

Sneaking up to Bobby's room, Joey tells her what happened on the day of his sister's death. They had argued and his sister had gone off to play on her own, wandering into the bathroom. Dropping her doll into the bath and slipping while trying to retrieve it, the little girl knocked herself unconscious. When Nanny returned that afternoon, she started routinely running a bath for the children, without noticing the unconscious girl. Coming back later and discovering the body, Nanny dementedly carried on bathing her as if she were still alive, and later prevented Joey from calling the police. Nobody believed Joey's version of events and suspicion fell on him.

Aunt Penny wakes up suddenly in the middle of the night and discovers Nanny outside Joey's bedroom door clutching a pillow. Nanny says she has brought it to help Joey sleep, but Penny remembers something from her childhood: that Nanny has never approved of pillows for children for fear they might cause suffocation. She realises that Joey's accusations against Nanny are true, but the shock of that knowledge triggers a fatal heart attack, Nanny refusing to bring the medicine that might save her. As Penny lies dying, Nanny remembers that fateful afternoon, when she was called out earlier to visit her own long-abandoned daughter, who is now dying from a botched abortion. In mental turmoil on returning to the house, Nanny abstractedly switched on the bathwater for the children without first checking where they were: tragedy has ensued.

Nanny breaks into Joey's room and, in his attempt to escape, he is knocked unconscious. Carrying him through into the bathroom, she starts to drown him, but at the last moment, pulls him to safety before suffering a complete mental breakdown. Later, a recovered Joey visits his mother in hospital and promises that he will look after her from now on.