We find out in the movie that her unusual fears have led her to steal. She worms her way into secretarial positions, becomes known as a responsible and caring person, and later steals from the business. She constantly changes location once the owners of a business where she steals from find out about her. Later on, she obtains a secretarial position with a publisher by the name of Mark Rutland, who falls madly in love with her.
Because Marnie can't handle being close to anyone, she steals from Mark but quickly is caught by him. He tells her she can marry him or go to prison. Of course, she chooses marriage but can't go through with the honeymoon and tries to commit suicide. In a few days, after Mark consoles her, she goes riding on her favorite horse. While riding her horse, she sees a red coat, and she becomes terrified and begins riding her horse in a wild frenzy which causes the death of the horse.
Later on, Mark becomes so concerned with her behavior that he forces her to go back to her mother's home in Baltimore. While there, Marnie remembers what happened to her when she was a small child. In her remembering, she recalls that her mother was a prostitute and that her leg got broken during a thunderstorm by a man who was her client. Marnie, wanting to protect her mother, killed the man by striking him on the head with a poker, causing a great deal of blood. Her mother took the blame for the crime, and that brought on tremendous guilt in Marnie. Once her fears come out, her relationship with Mark improves and life begins to look positive.