The film begins with a female voiceover: "Last night I dreamt I went to Manderley again", to images of a ruined country manor.
The heroine is a very young (and nameless) woman (Joan Fontaine), a paid companion to the wealthy but obnoxious Edythe Van Hopper (Florence Bates). The heroine meets the aristocratic widower Maximilian (Maxim) de Winter (Laurence Olivier) in Monte Carlo. They fall in love, and within two weeks they are married.
Maxim takes his new bride to Manderley, his country house in Cornwall, England. The housekeeper, Mrs. Danvers (Judith Anderson), is domineering and cold, and is obsessed with the great beauty, intelligence and sophistication of the first Mrs. de Winterthe eponymous Rebeccaand preserves her former bedroom as a shrine. Rebecca's sleazy cousin Jack Favell (George Sanders) appears at the house when Maxim is away.
The new Mrs. de Winter is intimidated by her responsibilities and begins to doubt her relationship with her husband. The continuous reminders of Rebecca overwhelm her; she believes that Maxim is still deeply in love with Rebecca. She also discovers that her husband sometimes becomes very angry at her for apparently innocent actions.
Trying to be the perfect wife, the young Mrs. de Winter convinces Maxim to hold a costume party as he did with Rebecca. The heroine tries to plan her own costume, but Mrs. Danvers suggests she copy the beautiful outfit in the portrait of Caroline de Winter, an ancestor. At the party, when the costume is revealed to Maxim he is appalled; Rebecca wore the same outfit at their ball a year ago, shortly before she died. The heroine confronts Danvers, who tells her she can never take Rebecca's place, and almost manages to convince her to jump to her death. A sudden commotion reveals that a ship is sinking.
The heroine rushes outside, where she hears that during the rescue a sunken boat has been found with Rebecca's body in it. Maxim admits that he had earlier misidentified another body as Rebecca's, in order to conceal the truth. At the very beginning of their marriage Rebecca had told Maxim she intended to continue the promiscuous and perverse sex life she had led before the marriage. He hated her but they agreed to an arrangement: she would act as the perfect wife and hostess in public, and he would ignore Rebecca's privately conducted affairs. Rebecca grew careless and complacent in her dealings, including an ongoing affair with her cousin Jack Favell. One night, Rebecca informed Maxim that she was pregnant with Favell's child. During the ensuing heated argument she fell, hit her head and died. Maxim took the body out in a boat which he then scuttled.
Shedding the remnants of her girlish innocence, Maxim's wife coaches her husband on how to conceal the mode of Rebecca's death from the authorities. In the police investigation, deliberate damage to the boat points to suicide. Favell shows Maxim a note from Rebecca which seems to indicate she was not suicidal. Favell then tries to blackmail Maxim, but Maxim tells the police. Maxim is now under suspicion of murder. The investigation then focuses on Rebecca's secret visit to a London doctor (Leo G. Carroll), which Favell assumes was due to her illicit pregnancy. However, the coroner's interview with the doctor reveals that Rebecca was mistaken in believing herself pregnant; instead she had a late-stage cancer.
The doctor's evidence persuades the coroner to render a finding of suicide. Only Frank Crawley (Maxim's best friend and manager of the estate), Maxim, and his wife will know the full story: that Rebecca lied to Maxim about being pregnant with another man's child in order to goad him into killing her, an indirect means of suicide. As Maxim returns home from London to Manderley, he finds the manor on fire, set alight by the deranged Mrs. Danvers. The second Mrs. de Winter and the staff manage to escape the blaze, but Danvers dies in the flames.