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 situation, though he makes it clear that Pete cannot be expected to live much longer.
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. When questioned by the manager, Martin presents himself as mentally challenged, and calls himself "Georgie". Apparently now disbelieving in a link between them, the manager asks Susan for her address, and Martin appears to make a mental note when she offers it. Sympathetic to him, Susan pays for the 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. Despite the apparent course of events in the toy shop, they have come to hear of the duck. There is allusion to some perverse behaviour he has exhibited, though this is not elaborated upon. In his room, now behaving as "Georgie", he rocks in a rocking chair while smiling meekly in the mirror and caressing a stuffed animal. The camera pans down to reveal that 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 a young Indian man named Shashee. He drops off Susan, who thanks him; 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 and returns to his room. Martin stares in the mirror, bare-chested, and caresses himself. He removes the rest of his clothes as the camera reveals a stack of bodybuilding magazines on his dresser. He then smashes the mirror in apparent frustration or anger.
Martin 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's room while cleaning it, and discovers several books hidden in Martin's drawer that a person with learning difficulties would not read or understand, as well as a book titled Knowing Yourself from Your Signature, in which signatures in the blank pages read '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.Hywel Bennett as Martin Durnley / Georgie
Hayley Mills as Susan Harper
Billie Whitelaw as Joan Harper
Phyllis Calvert as Enid Durnley
Frank Finlay as Henry Durnley
Barry Foster as Gerry Henderson
Salmaan Peer as Shashee Kadir
Christian Roberts as Philip Harvey
Gretchen Franklin as Clarkie
Thorley Walters as Sir John Forrester
Timothy West as Superintendent Dakin
Russell Napier as Professor Fuller
Timothy Bateson as Mr. Groom
Richard Davies as Taffy Evans
Basil Dignam as Doctor
Robin Parkinson as Shop manager
Marianne Stone as Store detective 1
John Harvey as Store Detective 2
Mollie Maureen as Lady patient
Brian Peck as Det. Sgt. Rogers
The film score was composed by Bernard Herrmann.
The theme can also be heard in Quentin Tarantino's Kill Bill when a menacing Elle Driver (Daryl Hannah) impersonates a nurse in the hospital scene and in Death Proof as Rosario Dawson's character's ringtone, in several episodes of American Horror Story, in the Malayalam movie Chaappa Kurish as a ringtone of Fahad Fazils character's iPhone and in the Bengali movie Chotushkone where also it is used as a ringtone for Parambrata Chatterjee's character's phone. More recently, it has also been used in Honda's 2015 car advertisement.
Stylotone Records reissued the score as part of a deluxe LP set, with a release date of 5 May 2016.
The film is notorious for its use of Down syndrome, then referred to as mongolism, as a catalyst for Martin's actions. The film opens with a spoken disclaimer of any connection between the disorder and antisocial behaviour. As The New York Times put it, "this is a delicate area indeed", going on to describe the film as, "more unsettling than rewarding, and certainly more contrived than compassionate".
The title comes from the poem Slaves by George Sylvester Viereck (1884-1962) which is quoted twice in the movie, once during Professor Fuller's lecture on chromosome damage, and then as an audio flashback when Martin/Georgie is in a cell:No puppet master pulls the strings on high
Proportioning our parts, the tinsel and the paint
A twisted nerve, a ganglion gone awry,
Predestinates the sinner and the saint.
Viereck's motives for his writing have been the subject of some discussion, and have further implications given the debate on eugenics during the middle of the 20th century, a subject somewhat alluded to in Professor Fuller's lecture in the film.