England, 1959. At the accounting firm of Crombie & Strutt where she works as a clerk, Marnie meets Mark Rutland, a handsome client of Mr. Strutt’s. Mark is immediately attracted to her. After the office closes, Marnie steals money from the office safe and as she escapes, plans how she will change her identity and looks when she moves on to the next town—as she has done before. Marnie visits her invalid mother and gives her the money for a new house. Meanwhile, Mr. Strutt discovers Marnie’s theft and vows to bring Marnie to justice.
Marnie applies for a job at Halcyon Printing and is shocked when the man who interviews her is Mark Rutland, whom she met at Mr. Strutt’s office. To Marnie’s relief, he seems not to recognize her and offers her the job. She also meets Mark’s brother Terry, Mark’s “wayward deputy” at the firm.
Weeks later, Terry, a notorious womanizer, invites Marnie to a poker game at his flat. When she joins her coworkers at a pub, they urge her to accept Terry’s invitation. A stranger appears, claiming to have known Marnie under a different name, but she dismisses him. Marnie joins Terry and his friends for the game. Afterwards, Terry makes a pass at Marnie, but she rebuffs him and escapes.
At Mark’s home, his mother, Mrs. Rutland, tells him to shape up as managing director. Marnie arrives, Mark having invited her on the pretext of work. Marnie mentions her beloved horse, Forio, and Mark speaks of his loneliness since his wife died. A thunderstorm approaches, terrifying Marnie. Mark comforts her, then declares his romantic fascination with her and tries to kiss her. Marnie quits her job on the spot and flees.
Marnie plans to escape the two brothers by changing identity once again. She breaks into the Halcyon safe, but Mark catches her in the act. He threatens to turn her in unless she agrees to marry him. Marnie has no choice but to comply.
Marnie’s mother receives a letter from Marnie, informing her that she’ll be out of touch for a while. She discusses her mistrust of Marnie with Lucy, her neighbor. Marnie’s mother believes that when Marnie was a girl, she killed her baby brother.
On their honeymoon cruise, Mark reveals to Marnie that he recognized her when she applied for the job and has known all along that she is a thief. She refuses sex with him, and he tries to rape her. She locks herself in the bathroom and slits her wrists.
Weeks later, Marnie removes the bandages from her wrists. The scars are fading, but she feels that her wounds will never heal and vows to remain strong and resist Mark.
Marnie and Mark dress for a business dinner. Marnie gives him information that leads him to conclude that Terry is scheming to take over the family firm. Frustrated by their sexless marriage, Mark presses Marnie to consult an analyst. In return, he promises to bring Forio to be stabled on his property. Marnie sees the analyst for several weeks and eventually recalls a memory of a thunderstorm, a soldier, her mother, and her dead baby brother.
Marnie and Mark arrive at the country club for his mother’s party.
Terry threatens to expose Marnie as a fraud. Mr. Strutt appears and recognizes Marnie, despite her denials. Mark agrees to meet with him later to resolve the situation. Terry excoriates Mark for his entrapment of Marnie, and the two men fight. Mrs. Rutland shocks Mark when she reveals that it is she who has been planning a takeover of the family firm, and now, Malcolm Fleet will run the business.
Marnie and Mark attend a fox hunt, with Marnie riding Forio. When the horse panics and bolts, Marnie is thrown and Mark, trying to help her, is injured. Forio is so badly hurt that he has to be shot. Mark’s mother visits him in the hospital, and expresses bewilderment at his marriage. Marnie arrives—the tension between her and Mark eased—but as she leaves, she steals his keys.
Deciding that she must leave the country to avoid her growing feelings for Mark, Marnie breaks into the office safe, but she finds herself unable to take the money. She goes to see her mother but arrives to find that she has died. At the cemetery, the mourners assemble. Lucy tells her that it was her mother, not Marnie, who killed Marnie’s brother. Marnie rejects her feelings of guilt and her need to transgress. Mark appears, accompanied by Terry and the police. Mark hopes that Marnie and he might reconcile, but Marnie can’t promise anything. She only knows that she must face the truth. She gives herself up to the police with the words, “I’m free.”