The film opens with Martin playing catch with his younger brother Pete, who has learning difficulties and lives in a segregated school in London. Martin is the only remaining figure in Pete's family life; their father died years ago and their mother has a new life with a new husband. Martin expresses concern for his brother's well-being to the school's physician, who is comfortable with Pete's progress.After the title sequence, Martin is shown in a toy store, gazing at Susan, who purchases 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 assures them she has never met Martin. The manager asks Susan for her address and Martin appears to make a mental note when she offers it. When questioned by the manager, Martin turns soft, presents himself as mentally challenged and calls himself "Georgie". Sympathetic to him, Susan pays for the toy. Certain 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. There is allusion to some perverse behaviour he has exhibited, though this is not elaborated upon. He shuts himself in his room. While secluded, Martin stares in the mirror, bare-chested, examining his frame. He seems disappointed in his appearance, eventually punching and cracking the mirror in frustration. The camera reveals a stack of bodybuilding magazines on his dresser.The next day, Martin goes to Susan's house and waits for her to return. She arrives with a young Indian man named Shashee. He drops off Susan, who thanks him and she goes to the library, where she keeps an after-school job. Martin approaches Susan who immediately recognises him as "Georgie". He tells her that he followed her and pays her back for the toy. Before he leaves, Martin, as Georgie, gets Susan to lend him a book 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 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 let him 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 he is in France. This series of decisions leads Martin down the path of self-destruction.One night, Martin sneaks out of Susan's house after stealing a pair of scissors and stabs his stepfather to death in the garage of his home after his stepfather comes home from a dinner party. The police investigate the next day and focus their attention to finding Martin for questioning.A few days later, Martin invites himself to tag along with Susan who is going for a swim at a country lake where Martin attempts to kiss her until she refuses his advances, making her uncomfortable and suspicious about him. At home a little later, Susan searches Martin room while cleaning it and discovers several books hidden in Martin's drawer that a simpleton person like him would not read or understand as well as book titled "Knowing Yourself from Your Signature" which the signatures in the blank pages list 'Martin Durney'.At this point, Susan begins investigating Martin, first by talking with his mother, and realizes that Martin and Georgie are one and the same after seeing a photograph of Martin at the house. Next, Susan visits Shashee at a hospital where he works as a resident to question him about split personalities and suspects that Martin may be not mentally challenged but a narcissistic sociopath.At Susan's house, Martin begins losing mental control over himself as he rightly suspects 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 (off-camera).When Susan arrives home, Martin holds her captive in his room after finally 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 just at the time that Shashee learns the truth about Martin and also calls the police from the hospital and races to the house to rescue Susan.The police arrive at Susan's house where they finally subdue and arrest Martin just when he appears that he is going to kill her. They burst into Susan's room as three shots are heard, but Martin had fired at his reflection in the mirror. As Martin is taken away he claims that he is Georgie and had killed Martin. Susan is unharmed but badly shaken. The final shot shows Martin, now confined in a cell at a local mental hospital, ranting over his lost love Susan.
Output: Question: Who is Martin Durney playing catch with?, Answer: Martin is playing catch with his younger brother Pete.