Plot
Martin plays catch with his older brother Pete, who has learning difficulties and has been sent to live in a special boarding school in London, by their mother. Martin is the only remaining figure in Pete's family life; their father died years before and their mother has a new life with her new husband, a wealthy banker. The school's physician believes that Pete cannot be expected to live much longer.
At a store, Martin later sees Susan purchase a toy. As she leaves, Martin follows after having pocketed a toy duck. Two store detectives ask them to return to the manager's office. The detectives assert that Martin and Susan were working together to allow Martin to steal a toy. Susan says she has never met Martin.
When questioned by the manager, Martin presents himself as mentally challenged, and calls himself "Georgie". Now disbelieving in a link between them, the manager asks Susan for her address, and Martin makes a mental note when she offers it. Sympathetic to him, Susan pays for Martin's toy. Implying that this was a misunderstanding, the manager lets them leave.
Martin returns home and finds his parents arguing in the parlor, over his lack of interest in life. They have come to hear of the duck incident. There is allusion to perverse behaviour he has previously exhibited. In his room, now behaving as "Georgie", he rocks in a rocking chair while smiling in the mirror and caressing a stuffed animal. The rocking motion of the chair is smashing a photo of his stepfather.
The next day, Martin goes to Susan's house and waits for her to return. She arrives with Shashee, a young Indian man. He drops off Susan, who thanks him and goes to the library, where she keeps an after-school job. Martin approaches Susan who recognises him as Georgie. Martin says that he followed her, and pays her back for the toy. Before leaving, Martin gets Susan to lend him The Jungle Book.
Martin has a dispute with his stepfather, who insists he travel to Australia. Martin refuses and returns to his room. Martin stares in the mirror, bare-chested, and caresses himself. He then removes the rest of his clothes. There are male bodybuilding magazines on his dresser. A frustrated Martin eventually smashes the mirror.
Martin sets in motion a plan to leave home, pretending to go to France. He then shows up late at Susan's mother's house, where she rents rooms. Presenting himself as Georgie, he gains sympathy both from Susan and her mother, and they let him stay. Martin wants Susan to accept him as a lover, but cannot reveal that he is in fact Martin, as he is worried she will shun him.
One night, Martin sneaks out of Susan's house after stealing scissors, and stabs his stepfather to death after the latter leaves a dinner party. The police investigate the murder and focus their attention to finding Martin for questioning.
Days later, Martin invites himself to tag along with Susan who is going for a swim at a country lake. There, Martin attempts to kiss Susan, who refuses his advances, feeling uncomfortable and suspicious about him. Later at home, Susan searches Martin's room while cleaning and discovers books hidden in a drawer that a person with learning difficulties would not read or understand, as well as a book titled Know Yourself from Your Handwriting, in which signatures in the blank pages read 'Martin Durnley'.
Susan begins investigating Martin, first by talking with his mother, and realizes that the two brothers are one and the same after seeing a photograph of Martin at the house. Susan visits Shashee at a hospital where he works as a resident to question him about split personalities.
At Susan's house, Martin begins losing control over himself while suspecting that Susan may know who he really is. When Susan's neglected and unsuspecting mother attempts to sexually arouse Martin, he kills her by hacking her apart with a hatchet in the backyard wood shed.
When Susan arrives home, Martin holds her captive in his room after revealing his true persona. He forces Susan to undress so he can sexually fondle her, while Susan's mother's body is found in the woodshed by Gerry Henderson, one of the "paying guests", who calls the police at the time that Shashee learns the truth about Martin and races to the house to rescue Susan.
The police arrive at Susan's house and subdue Martin before he kills her. They burst into Susan's room as three shots are heard and learn that Martin fired at his reflection in the mirror. While being taken away, he claims that he is Georgie and has killed Martin. Susan is unharmed but badly shaken. Martin is confined in a cell at a mental hospital, ranting over his lost love Susan.
