The film begins with a mysterious woman with dark hair, only seen from behind, as she navigates a train platform carrying a suitcase. Her accompanying yellow purse, held under her arm, strikingly resembles female genitalia in its positioning.
Shortly after, we see a furious business owner, Sidney Strutt, who vents his frustrations to two detectives about being "cleaned out" by a stunning employee named Marion Holland. Strutt, clearly drawn to her looks despite her lacking professional credentials, is overheard by a client, Mark Rutland, who humorously refers to her as "the brunette with the legs."
The woman, known in reality as Marnie Edgar, is a compulsive thief who has made off with nearly $10,000 hidden in that very yellow purse. Changing her appearance from brunette to blonde, she escapes with the stolen cash and heads south to Virginia, where she keeps her cherished horse, Forio. During this journey, she visits her mother, Bernice, in Baltimore, showering her with luxurious gifts in an effort to gain her affection. However, Bernice remains cold and distant, displaying more warmth towards a local girl named Jessie, further intensifying Marnie's feelings of jealousy and inadequacy. Marnie’s odd fixation on Jessie and her anxiety over a bouquet of red gladiolas hints at the deep-seated dysfunction driving her lies and thefts.
As time passes, Marnie adopts a new identity—Mary Taylor—with ambitions to scam yet another innocent victim. She secures a job at a printing company located in Philadelphia, owned by none other than Mark Rutland himself. Although now with chestnut hair, he recognizes her as the thief from Strutt's office. Instead of alerting the police, he is captivated by the thrill of pursuing the attractive criminal and offers her a position. True to her nature, Marnie attempts to rob Rutland, but he has foreseen her move and tracks her down instead. Rather than turning her in, he resorts to blackmail, insisting she marry him.
During their honeymoon on a cruise ship, Mark discovers Marnie's intense aversion to intimacy. Initially, he respects her boundaries; however, his obsession with control and dominance leads to a harrowing incident of sexual assault. Following this, Marnie, overwhelmed by despair, attempts suicide by drowning in the ship’s pool, but Mark intervenes just in time to save her.
Challenges arise for their marriage, notably from Lil, the sister of Mark’s deceased first wife, who harbors ambitions to reclaim his affection and views Marnie as a rival. Learning that Mark has resolved Strutt's silence concerning Marnie's crime by paying the stolen money, Lil invites Strutt to a gathering at their home. Strutt, recognizing Marnie as the woman who outsmarted him, remains silent about her past actions when faced with Mark's threats of withdrawing his business.
Meanwhile, Marnie embarks on a fox hunt with Forio, but the frenzied scene triggers a deep-rooted panic response in her. She loses control of her horse as it charges towards a high stone wall, resulting in a tragic accident where Forio suffers severe injuries. In a heart-wrenching decision, she must put him down to relieve his suffering, causing her already fragile mental state to shatter further.
With emotions running high and a gun in hand, she is tempted to revert to her thieving ways against Mark. Yet, his strategic maneuvers thwart her compulsive instincts, plunging Marnie into a tumult of conflicting desires. Mark, determined to understand Marnie's haunting behaviors—her bizarre fears of thunderstorms, men, and the color red—takes her back to her mother’s home in Baltimore.
There, in a moment of catharsis, Marnie relives a disturbing memory from her childhood involving her mother’s visitors and a tragic event that would forever scar her psyche. Bernice explains her own troubled past, including the circumstances surrounding Marnie’s birth and the protective love she had for her daughter, ultimately revealing the source of Marnie's haunting fears.
Freed from the shadows of her repressed memories, Marnie believes she can finally abandon her criminal past and embrace a genuine relationship with Mark. The film concludes with the couple riding back to his family estate, with Marnie hopeful for a fresh start.
