Rebecca is the story of a nameless young woman who marries the wealthy and urbane Mr. de Winter in the wake of the death of his first wife, the titular Rebecca. The new Mrs. de Winter is tormented at her husband's mansion, Manderley, by the cruel and manipulative housekeeper Mrs. Danvers. The novel deals with emotional abuse, the nature of relationships, the fears faced by young wives, and with the obsessive cult of personality the late Rebecca constructed for herself.
While working as the companion to a rich American woman on holiday in Monte Carlo, the unnamed narrator, a naïve young woman in her early 20s, becomes acquainted with a wealthy Englishman, Maximilian (Maxim) de Winter, a widower aged 42. After a fortnight of courtship, she agrees to marry him and, after the wedding and honeymoon, accompanies him to his mansion in Cornwall, the beautiful West Country estate Manderley.
Mrs. Danvers, the sinister housekeeper, was profoundly devoted to the first Mrs. de Winter, Rebecca, who died in a boating accident about a year before Maxim and the second Mrs. de Winter met. She continually attempts to undermine the new Mrs. de Winter psychologically, subtly suggesting to her that she will never attain the beauty, urbanity and charm her predecessor possessed. Whenever the new Mrs. de Winter attempts to make changes at Manderley, Mrs. Danvers describes how Rebecca ran it when she was alive. Each time Mrs. Danvers does this, she implies that the new Mrs. de Winter lacks the experience and knowledge necessary for running an important estate. Cowed by Mrs. Danvers' imposing manner, the new mistress simply caves in.
She is soon convinced that Maxim regrets his impetuous decision to marry her and is still deeply in love with the seemingly perfect Rebecca. The climax occurs at Manderley's annual costume ball. Mrs. Danvers manipulates the protagonist into wearing a replica of the dress shown in a portrait of one of the former inhabitants of the estate—the same costume worn by Rebecca to much acclaim shortly before her death. The narrator has a drummer announce her entrance using the name of the lady in the portrait: Caroline de Winter. When the narrator shows Maxim the dress, he gets very angry at her and orders her to change.
Shortly after the ball, Mrs. Danvers reveals her contempt for our heroine, believing she is trying to replace Rebecca and reveals her deep, unhealthy obsession with the dead woman. Mrs. Danvers attempts to take revenge by encouraging Mrs. de Winter to commit suicide by jumping out of the window. However, she is thwarted at the last moment by the disturbance occasioned by a nearby shipwreck. A diver investigating the condition of the wrecked ship's hull also discovers the remains of Rebecca's boat, with her body still on board.
Maxim confesses the truth to our heroine: how his marriage to Rebecca was nothing but a sham; how from the very first days husband and wife loathed each other. Rebecca, Maxim reveals, was a cruel and selfish woman who manipulated everyone around her into believing her to be the perfect wife and a paragon of virtue. She repeatedly taunted Maxim with sordid tales of her numerous love affairs. The night of her death, she told Maxim that she was pregnant with another man's child, which she would raise under the pretence that it was Maxim's and he would be powerless to stop her. In a rage, he shot her, then disposed of her body on her boat and sank it at sea. The second Mrs. de Winter thinks little of Maxim's murder confession but instead is relieved to hear that Maxim had never loved Rebecca but instead loves her .
Rebecca's boat is raised and it is discovered that it was deliberately sunk. An inquest brings a verdict of suicide. However, Rebecca's first cousin (and lover) Jack Favell attempts to blackmail Maxim, claiming to have proof that Rebecca could not have intended suicide, based on a note she sent to him the night she died.
It is revealed that Rebecca had had an appointment with a Doctor Baker in the outskirts of London shortly before her death, presumably to confirm her pregnancy. When the doctor is found, he reveals that Rebecca had been suffering from cancer and would have died within a few months. Furthermore, due to the malformation of her uterus, she could never have been pregnant. Maxim assumes that Rebecca, knowing that she was going to die, manipulated him into killing her quickly; Mrs. Danvers had said at the inquiry that Rebecca feared nothing except dying a lingering death.
Maxim feels a great sense of foreboding and insists on driving through the night to return to Manderley. However, before he comes in sight of the house, it is clear from a glow on the horizon and wind-borne ashes that it is ablaze.