In London, twenty-two year-old Peggy Heller (Judy Geeson) meets and marries school teacher Robert Heller (Ralph Bates) after recovering from a nervous breakdown. Robert works in the countryside in a private school owned by headmaster Michael Carmichael (Peter Cushing), who is married to Molly Carmichael (Dame Joan Collins). On the eve of moving to the country with her husband, Peggy spends the night at the boarding house of Mrs. Beamish (Gillian Lind) and is attacked by a man with a mechanical arm in her room. Mrs. Beamish calls the doctor, but they do not believe Peggy. The next morning, she heads with Robert to the country and moves into the cottage in the school. Soon, Peggy is attacked by the same man, but Robert does not believe her. Then she meets the Headmaster and realizes that he has a mechanical arm. What will happen next?