The film opens with Martin playing catch with his young mentally challenged brother Pete at a special school in London. We learn that Martin is the only thread to his brother's family life, his father having died years before, and his mother having moved on to a new life with her new husband. Martin expresses his concern for his brother's well-being to the school's physician, who asserts his comfort with Pete's progress.
After the title sequence, Martin is shown in a toy store where he is gazing at Susan, who purchases a toy. She leaves, but Martin follows. As they leave the store, two undercover store detectives stop them and ask them to return to the store manager's office. There he questions them, and the detectives assert that Martin and Susan were working together to allow Martin to steal a small toy duck. Susan assures the three that she has never met Martin. The manager asks Susan for her address, she offers it, and Martin appears to make a mental note of this. When the store manager questions Martin, he turns soft, and presents his self as mentally challenged, referring to himself as "Georgie." Susan, sympathetic to the young man, pays for the toy. Sure that there has been a great misunderstanding, the store manager allows them to leave.
Martin returns home to his parents arguing in the parlor over his lack of interest in life. There is allusion to some perverse behavior he has exhibited, though this is not elaborated upon. He sneaks upstairs and shuts himself in his room. While he is secluded, Martin stares at him self in his full length mirror, bare chested, examining his frame. He seems to be disappointed at his appearance, eventually punching and cracking the mirror in frustration. The camera pulls back to reveal a stack of body building magazines on Martin's dresser.
The following day, Martin goes to Susan's house and waits for her to return. She arrives with a young Indian man, who is later identified as Shashee. He drops Susan off, who thanks him, and she makes her way to the library where she keeps an after-school job. There she is peered at by two 10-year-old boys who are presented a book by her to read. When they leave, Martin approaches Susan who immediately recognizes him as 'Georgie.' He tells her that he followed her and then pays her back for the toy. Before he leaves, Martin, as Georgie, gets Susan to lend a book out to him about animals.
Martin has a heated conversation with his stepfather, who insists he travel to Australia. Martin refuses, then sets in motion a plan to leave his home, pretend to go to France and then go on to live with Susan. Martin leaves his family and 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 allow him to stay.
The plot unravels with Martin's duplicitous nature clashing against his desires to win Susan's heart. He wants her to accept him as a lover, but cannot reveal that he is in fact Martin, as he is worried she will shun him. Meanwhile, Martin uses his new found identity to his advantage to seek out revenge on his stepfather, who believes him to be in France. This series of decisions leads Martin down the path of self-destruction.