Mark marries Marnie although she is a habitual thief and has serious psychological problems, and tries to help her confront and resolve them.
Marnie Edgar is a habitual liar and a thief who gets jobs as a secretary and after a few months robs the firms in question, usually of several thousand dollars. When she gets a job at Rutland's, she also catches the eye of the handsome owner, Mark Rutland. He prevents her from stealing and running off, as is her usual pattern, but also forces her to marry him. Their honeymoon is a disaster and she cannot stand to have a man touch her, and on their return home, Mark has a private detective look into her past. When he has the details of what happened in her childhood to make her what she is, he arranges a confrontation with her mother realizing that reliving the terrible events that occurred in her childhood and bringing out those repressed memories is the only way to save her.
In her own words, the beautiful Marnie Edgar is a cheat, a liar, and a thief. Her M.O.: using one of several aliases, each with a distinct social security number and a different look (generally entailing a different hair color and style), she gets an accounting type job at an office which will allow her access to the office safe, steals the money in the safe, then disappears without a trace. Much of that money goes to her Baltimore port area residing mother, Bernice Edgar, who doesn't ask from where the money comes and who seems to have an uneasy relationship with her daughter. The latest scam on which Marnie embarks, using the alias Mary Taylor, a widow, is at Philadelphia based Rutland & Co. What Marnie is unaware of is that company President Mark Rutland, a recent widower, who saw Marnie when she came in for her interview, suggested that she be hired despite she not being the most qualified candidate, solely because he recognized her as the beautiful Marion Holland, the former clerk at Strutt & Co., a client of Rutland, who was hired there solely for her good looks, and who reportedly stole ten thousand dollars from its office safe. Marnie does not recognize Mark. Mark wants to find out her story. He begins to do so after she manages to steal money out of the Rutland office safe. Mark did some research on her and she, who is between a rock and a hard place concerning this most recent theft, eventually fills in the rest of the details concerning her criminal activities. However, Mark doesn't send her to prison, but rather has a proposition for her which she can't refuse. This agreement with Mark ends up being her own kind of prison, from which she tries to escape. While Mark tries to understand why Marnie is the way that she is, which includes seemingly morbid fears of thunderstorms, saturated instances of the color red, and being touched by men, Mark's sister-in-law, Lil Mainwaring, who seems to have aspirations to be the next Mrs. Rutland, tries to find out what Marnie and Mark are hiding. If Lil is successful, Marnie's past could catch up with her and Mark and their arrangement.
Marnie Edgar is an ice-cold habitual thief. She uses her looks to gain the confidence of her employers, robs them, and changes her identity. Her only loves are her horse and her mother, although she has problems with the latter relationship. Marnie applies for a job at Mark Rutland's Philadelphia publishing company. Mark recognizes her since he is a client of her last employer, but instead of turning her in, he decides to watch her.
The beautiful Marnie Edgar is a first-class bookkeeper and the apple of her employers' eyes. She happens also to be a compulsive thief who has taken her last employer, Sidney Strutt, to the cleaners before disappearing into thin air. But with Mark Rutland, she attempts robbery once too often. He forces her to marry him, and out of love for her, uncovers the mysteries of her past that contribute to her penchant for taking other people's money.
- 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.