The story opens as a woman with dark hair, seen only from behind, carries a suitcase along a train platform. The yellow purse tucked under her arm is presented at such an angle that it is suggestive of female genitalia.In the next scene, outraged business owner Sidney Strutt (Martin Gabel) rants to a pair of detectives that he was "cleaned out" by a female employee named Marion Holland whom he obviously hired for her looks despite her lack of professional references. A client of Strutt's, Mark Rutland (Sean Connery), overhears the conversation and says that he remembers this woman, wryly referring to her as "the brunette with the legs."The woman, real name Marnie Edgar (Tippi Hedren) is a compulsive thief, and has made off with almost $10,000 stashed in the yellow purse. She changes her appearance, going from brunette to blonde, and flees with the cash, journeying south to a town in Virginia where she keeps her beloved horse, Forio, stabled. She then visits her mother Bernice (Louise Latham) in Baltimore. She lavishes her mother with expensive gifts in an attempt to impress her and win her love, but the woman is cold and distant with Marnie, showing much more maternal affection towards a little neighbor girl named Jessie whom she babysits. Marnie's intense jealousy of Jessie seems inappropriate for a grown woman to express, and along with her panic over a bouquet of red gladiolas and her mother's insistence that "decent" women don't need men, the first hints of the dysfunction that drives Marnie to lie and steal appear.Time goes by and Marnie constructs a new identity, Mary Taylor, with the intent to run her scam again on another victim. She applies for a job at a printing company in Philadelphia owned by none other than Mark Rutland. Despite her now chestnut hair, he recognizes Marnie as the woman who stole from Strutt, but, excited by the thrill of chasing and capturing the attractive criminal, he hires her. Marnie robs Rutland, too, but he anticipated this outcome and manages to track Marnie down. Instead of handing her over to the police, he blackmails her into marrying him.On their honeymoon, which takes place aboard a cruise ship, Mark finds out about Marnie's frigidity. At first, he respects her wishes, but soon obsessed with controlling and dominating Marnie, he rapes her. The next morning, she tries to commit suicide by drowning herself in a pool aboard the ship, but Mark finds her in time to resuscitate her.In addition to Marnie's aversion to sex and generally troubled behavior, Mark's marriage faces a challenge from Lil (Diane Baker), the sister of his deceased first wife who clearly had designs on him and views Marnie as a rival she can overthrow. Upon learning that Mark has bought Strutt's silence by paying off the money Marnie stole, Lil invites Strutt to a party at the house. Strutt recognizes Marnie, the former "Marion Holland" who outfoxed and humiliated him, but grudgingly keeps quiet about her crime when Rutland threatens to take his business elsewhere and convince other clients to do the same.Meanwhile Marnie takes Forio out on their first fox hunt. As the hounds descend upon the fox in a snarling pack, Marnie sees the bright red jacket of one of the hunters; the combination of the violent scene and the color triggers Marnie's own fight-or-flight response. She charges away into the woods at a mad gallop. She loses control of Forio and can't rein him in when he heads for a high stone wall. The horse breaks both of his front legs against the wall and crashes to the other side in agony. Marnie is forced to shoot Forio to put the animal out of his misery, an act that ruptures her already fragile state of mind. Still armed with the gun she used to kill her horse, she heads for the Rutland safe. She's driven to repeat her usual modus operandi-- steal from Mark and run away, but by putting Marnie in a position where she already owns anything she might ordinarily be tempted to steal, Mark has frustrated her compulsive desires. Marnie breaks down in a welter of neurotic conflict.Mark attempts to discover the root of Marnie's compulsions, nightmares and odd phobias: of thunderstorms, of men and of the color red. He takes Marnie to her mother's house in Baltimore where he demands an explanation of the "bad accident" Bernice suffered. He knows from reading reports of the incident that Bernice made her living as a prostitute whose main clientele were the sailors that frequented Baltimore's shipyards. Marnie relives the night when she was six years old and one of her mother's clients (Bruce Dern) approached Marnie as she cowered on the sofa, frightened by a thunderstorm. Bernice reacted to the sight of the drunken sailor caressing her daughter by attacking him frantically. Seeing her mother struggling with the man, Marnie struck him on the head with a fireplace poker, killing him. The blood spouting from his head wound to drench his white uniform led to her fear of the color red.Bernice explains that she became pregnant with Marnie as a teen after she'd been lured into having sex by the promise of her date's basketball sweater, and tells her daughter that she claimed responsibility for the sailor's death and fought to keep the authorities from placing Marnie in a foster home because Marnie's the only thing she ever really loved.Relieved of the repressed traumatic memories that poisoned her subconscious, Marnie believes that she is capable of renouncing crime and sustaining an intimate relationship. She and Mark ride back to his family estate.