Years after her aunt was murdered in her home, a young woman moves back into the house with her new husband. However, he has a secret that he will do anything to protect, even if it means driving his wife insane.
After the death of her famous opera-singing aunt, Paula is sent to study in Italy to become a great opera singer as well. While there, she falls in love with the charming Gregory Anton. The two return to London, and Paula begins to notice strange goings-on: missing pictures, strange footsteps in the night and gaslights that dim without being touched. As she fights to retain her sanity, her new husband's intentions come into question.
Paula's aunt, Alice Alquist, a famous entertainer, is murdered in her home. Paula, who lives with her aunt, finds the body. Police fail to find the killer, and Paula is sent away to school. Ten years later, Paula returns to London with her new husband. They take up residence in her aunt's house, which she has inherited. Paula is increasingly isolated by her husband but does come to the attention of an admirer of her aunt, Mr. Brian Cameron.
In London, at 9 Thorton Square, the prima donna Alice Alquist is strangled, and her famous jewels disappear. Her young niece Paula is sent to Italy to study music, and the house stays empty. Ten years later, Paula decides to get married with an older pianist named Gregory Anton, who convinces her to move back to the old address in London. When they arrive, Paula finds a letter from a mysterious and unknown Sergis Bauer, which makes Gregory upset. He psychologically begins to torture Paula, and she has a nervous breakdown and also has insecurity and memory problems. When the Scotland Yard policeman Brian Cameron sees Gregory and Alice in a place popular with tourists, he immediately he sees Paula, who reminds him of her aunt, but he does not know Gregory, and decides to investigate and find evidence to connect Gregory with the unsolved murder, while Paula is being driven insane and menaced by her husband.
- Why does the flame go down? Lights in the London house are from fixtures with gas flames, and when you light one light, it reduces gas supply to the other lights in the house that are close by, and the light dims. Yet no one in the house has lit any other lights! And there are also footsteps overhead, from a nailed-closed attic. Neither of the two servants sees or hears either of these signs. Paula Anton (Ingrid Bergman) thinks she is losing her mind, just as she has lost the brooch her husband Gregory (Charles Boyer) gave her.
Her new marriage is falling apart; she cannot go out lest she make another embarrassing scene. Is it the house, where Paula's aunt, a famous and beautiful concert singer, was murdered when the young Paula resided there? What does her new husband, who plays the piano beautifully, do for a living? Nothing. Why does he go out every night and leave her alone to fret and worry?
Who is the man who sees them at unexpected times and places, a man we soon learn is Scotland Yard detective Brian Cameron (Joseph Cotton)? Cameron is curious about the unsolved murder of Aunt Alice Alquist, who looked a great deal like Paula does now -- a murder that defied the investigators. No motive, no suspects, no clues.
You now have the clues to this Oscar-winning (Best Actress) dark mystery. Introducing (first picture) Angela Lansbury (Best Supporting Actress nominee) in the role of one of the servants. Also nominated for Best Picture, Best Actor (Boyer), and three more.