Julia Lofting (Mia Farrow), her husband Magnus (Keir Dullea), and their young daughter Kate (Sophie Ward) eat breakfast and Kate starts to choke on a piece of an apple. When the apple refuses to dislodge, Julia reaches for a knife and tries to perform a tracheotomy on Kate's throat, but fails and Kate bleeds to death.Distraught by the death of her only child, Julia leaves her husband and moves to a new house; a mysterious old row house in a low part of London. An upstairs room contains a child's possessions. As she unpacks, Julia finds a mechanical clown toy with sharp, clanking cymbals and accidentally cuts her finger on them. In a nearby park, Julia thinks she sees Kate, but the girl disappears. She hears strange noises in her house, blaming them on Magnus, who she believes is spying on her. When Julia takes photos of the child's possessions in the room, she is baffled when there is nothing in the frame of the developed photo and sees that the items cannot be photographed. A radiator in the house mysteriously turns on by itself. Later, Julia sees the girl in the park, but finds a mutilated turtle and a knife where she stood.Magnus' sister, Lily, asks Julia if her friends can use her house to conduct a seance. The medium, Mrs. Fludd, explains that spirits need to control someone to carry out physical acts. During the seance, Mrs. Fludd becomes frightened and tells Julia to leave the house immediately. Moments later, one of Lily's friends, Miss Pinner, falls down the stairs.While Julia is out, Magnus breaks into her house to find her. He sees something and follows it to the basement, but is killed after falling from the staircase - his throat cut on a broken bottle.A neighbor tells Julia the house once belonged to Heather Rudge, who moved away after her daughter Olivia choked to death (in a similar manner to Kate). Julia visits Mrs. Fludd, who has taken ill since the seance. She tells the medium she is convinced her house is possessed by Kate's spirit, but the medium tells her she saw a boy in the park, bleeding to death. When Julia finds an article about a boy, Geoffrey Braden, murdered in the park, she visits Greta Braden, Geoffrey's mother. Mrs. Braden tells her a vagrant was executed for the crime but that the other children in the park murdered her son. She says she and her companion have been following the lives of the children, now adults, and asks Julia to visit the remaining two, Captain Paul Winter and David Swift. Winter dismisses her after she says she's seen Mrs. Braden. Outside she sees Miss Pinner, who denies seeing anything frightening in the house. Julia next visits Swift, an alcoholic, who admits that Olivia had power over him and the other children. She made them each kill an animal, then made them watch as she murdered Geoffrey. He says he told only Mrs. Rudge.That night, as she sleeps, a child's hand caresses Julia's face and holds her hand. The phone rings and when Julia tries to answer it, a child's hand pushes hers away, causing her water glass to fall and break. Later, Swift is shown leaving his apartment. He slips on a broken bottle in the stairwell and he falls over the railing to his death.Julia tells her friend, Mark (Tom Conti) what she has discovered but he doesn't believe her. That evening, her heater won't turn off. She tries to unplug it and shocks herself. Mark is later electrocuted when a lamp falls into his bath. Julia visits Mrs. Rudge in a psychiatric home. Mrs. Rudge confesses that she killed Olivia after learning about the murder. As Julia rushes out in fear, she looks over her shoulder at Mrs. Rudge, who sees Olivia's eyes. Mrs. Rudge dies of a heart attack.At home, Julia sees the ghost Olivia in the bathroom mirror, then in the living room on the floor playing with the clown toy. She takes it from her, offers her a hug and asks her to stay. In the final shot, Julia is seen bleeding to death, her throat apparently cut on the sharp edges of the cymbals.