A young woman named Marnie Edgar, who has a habitual pattern of embezzlement and theft, lands secretarial jobs under fake identities. She steals from her employers and then disappears, changing her appearance and identity to remain untraceable. Marnie's latest victim is Mark Rutland, a widower and wealthy owner of a publishing company. Mark becomes intrigued by Marnie's behavior, particularly her extreme reactions to the color red and thunderstorms, which suggest a deeper psychological trauma.

Mark confronts Marnie with evidence of her theft, but rather than turning her over to the police, he coerces her into marrying him. He is motivated both by his attraction to her and by a desire to understand her compulsions. The marriage, however, is fraught with tension and complexity, as Marnie remains cold and distant towards Mark. Despite her resistance, Mark works to uncover the mysteries of her past, believing that this will help her overcome her psychological issues and the events that trigger her criminal impulses.