Peggy Heller Judy Geeson, a quiet 22-year-old caregiver, has just married Robert Heller Ralph Bates and is set to move with him to a secluded boys' boarding school south of London where he works. The night before they are due to depart, she is attacked in her home by a one-armed man with a prosthetic hand and falls unconscious. The chilling assault leaves her shaken, and the next morning she and Robert set off for the remote school.
Upon arrival, they settle into a chalet opposite the main building and plan to dine with the Carmichaels, headmaster Michael Carmichael Peter Cushing and his wife. The following day, Robert goes to work while Peggy explores the sprawling, once-grand campus; the hallways echo with the distant voices of boys, yet the classrooms feel strangely empty. A tour with the headmaster reveals more of the school's faded glory, and just as Peggy begins to feel at ease, she is again assaulted. Robert returns home worried about Peggy's mindset, but he remains skeptical of her account, canceling the dinner with the Carmichaels.
As they wander the grounds, Peggy and Robert encounter Molly Carmichael Joan Collins, the headmaster's wife, who is rabbit-hunting and appears cool and distant toward Peggy. That evening, with Robert away in London for a meeting, Peggy suspects an intruder in the house and arms herself with a shotgun. Descending the stairs, she sees Michael entering with a prosthetic arm, and in fear she fires at him. Michael escapes, but Peggy is chased through the building, where a chorus of boys' voices seems to mock the pursuit. Cornered in a dormitory upstairs, she fires again, but Michael remains unmoved and she faints.
The next day, Robert returns to find Peggy in a near-catatonic state and a pool of blood on the hallway floor; Michael is nowhere to be found. Peggy insists she cannot remember what happened. Robert explains that he once met Michael when he was a medical student, and that years earlier the school nearly burned down. Devastated by that disaster, Michael returned to the property and began re-creating the school’s former atmosphere by looping recordings of boys’ laughter and classroom lectures over the intercom.
That night, Robert meets with Molly and it is revealed they are having an affair, and that Robert married Peggy to compel her into killing Michael out of fear for her life. Bringing Peggy forward, Molly demands to know where Michael’s body is. The sound of bells fills the halls as Robert binds Peggy and leads her into the main hall, where Michael’s voice crackles from the intercom. Michael reveals he knew about the plot to kill him and that the shotgun in the chalet was loaded with blanks. Robert then loads the gun with real bullets and shoots what he believes to be Michael under a sheet on a couch. When the sheet is lifted, Molly’s dead body—a bound and gagged figure—is revealed instead.
Robert storms from the school with Peggy, attempting to hang her from a tree in a staged suicide. A sudden, vicious grip from Michael stops him, and the next morning two policemen arrive after receiving a call from the now-present Michael. Peggy tells them he is inside the building and that a new term is about to begin, but one officer notes the school has been shut down for years. A distant, eerie boys’ choir begins to echo through the structure, and in the tree behind the school hangs Robert, lifelessly strung up in a final, grim twist.
