Gail Cooper, Holly Palance, travels to her ex-husband Nick's apartment in London to retrieve some of its more valuable items. It quickly becomes clear that Nick isn't at home, but someone else is lurking inside. The unseen observer watches as Gail answers a phone call from a reporter about the couple's divorce and asks when Nick will release his next album, noting that he has been on a six-year hiatus since marrying Gail. Soon after, as Gail prepares to leave, she is attacked by a killer wearing a hag mask and a lace shawl, who uses a sickle to end her life in a brutal, shocking moment.
Nick Cooper, Jack Jones, a successful singer, arrives in London from New York, where he has been attempting to record a new album for his manager Webster Jones David Doyle. He finds some solace in Webster's secretary Linda Everett Pamela Stephenson, with whom he shares a strong mutual attraction. After a period of hesitation, Nick moves into a manor in the Surrey countryside that is overseen by the housekeeper Mrs. B Sheila Keith and the gardener Mr. B Bill Owen. Soon after, Nick begins to experience strange phenomena, including visions of Gail, even though he is unaware of her death, which leaves him confused about the haunting presence in his life. Despite this, he starts working on his album and grows closer to Linda, whose past with Webster adds another layer of tension to his new romance.
Nick's associate Harry Cunningham Peter Turner goes missing, heightening the sense that something is terribly wrong. Nick also learns that Linda had previously dated Webster, a detail that unsettles him as the unusual events intensify. One night, driven by curiosity, Nick investigates the noises he's heard and discovers Gail's severed, decomposing head, a shocking revelation that thrusts him into a catatonic state and leads to a brief hospital stay for his wellbeing.
As he recovers, Nick and Linda deepen their bond, but Linda disappears the following day. Dr. Paulsen advises Nick to slow down and revisit his old apartment, suggesting that the earlier uncanny occurrences are tied to his distress over the divorce and the stress of the recording project. Returning to the London residence, Nick notices the place has been thoroughly cleaned with antiseptic, and the carpeting has been replaced—an odd change since he left it in a different condition and had given no instructions for renovation. Mrs. B counsels Nick not to worry, but the unsettling atmosphere only grows.
The terror escalates when the masked interloper attacks again. In a tense confrontation, Mr. B reveals that the masked old woman is actually him, explaining that they have killed Gail and Harry in a calculated response to Nick's life and choices. Their aim was to drive him insane as vengeance for their daughter's obsession with him and her subsequent suicide after his marriage to Gail. The confrontation ends with Mr. B attempting to finish Nick off, but Nick manages to duck, and Mrs. B suffers the fatal wound, collapsing while cradling her dead husband’s lover’s body. Webster arrives and, upon witnessing the horror, calls the police. Just before the officers arrive, Nick hears tapping in the walls and uncovers Linda entombed inside the walls alongside the body of their dead daughter. They escape and head outside, where the police cart away Mr. B. In the final eerie moment, Nick sees the ghost of Gail waving at him from a manor window, suggesting that some, if not all, of the strange phenomena were real.
