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 Alquist (Ingrid Bergman) 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 (Charles Boyer). 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.
It's the late nineteenth century. Paula Alquist (Ingrid Bergman), an aspiring opera singer, and Gregory Anton (Charles Boyer), the accompanist at her singing lessons under the tutelage of Maestro Guardi (Emil Rameau) in Italy, fall in love and get married. Before learning of her background, Gregory is able to convince her to live in London, then when she divulges to him that she owns a house there at 9 Thornton Square, it bequeathed to her by her famous maternal aunt, opera singer Alice Alquist, who raised Paula and whom Paula found murdered in the house, the murder never solved. Not having thought about the house in a long time, Paula believes Anton's love can help her through whatever pain she may have felt about her aunt's murder now to live there with him. Over time at the house, Paula begins to forget things, lose things, and imagine things - such as an animosity with the new Cockney housekeeper Nancy (Angela Lansbury), who has a penchant for making "friends" with whatever Police Constable is patrolling the neighborhood - with Gregory quietly convincing her, without using the exact words, that she is ill. As such, she becomes isolated in the house, which only exacerbates her feeling that she indeed is going mad. However, two people on the outside believe that funny things are happening behind the closed doors of 9 Thornton Square, where only the domestics, Elizabeth (Barbara Everest) and Nancy, are allowed in, and only Gregory, who ever leaves by himself to take a walk every night. Those two are: Miss Thwaites (May Whitty), an elderly busybody neighbor who loves a good mystery; and Brian Cameron (Joseph Cotten), a fan of Alice Alquist, and who, in his professional life, arguably knows the most about her murder besides the actual murderer.
Paula Alquist's (Ingrid Bergman'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 (Joseph Cotten).
In London, at 9 Thorton Square, prima donna Alice Alquist is strangled, and her famous jewels disappear. Her young niece Paula Alquist (Terry Moore) is sent to Italy to study music, and the house stays empty. Ten years later, Paula (Ingrid Bergman) decides to get married with an older pianist named Gregory Anton (Charles Boyer), 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 Scotland Yard Police Constable Brian Cameron (Joseph Cotten) 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.