Two criminals stalk the streets of London one dark night. Frank Clemmons, [Dirk Bogarde], uses a gun to hold up psychiatrist Dr. Clive Esmond, [Alexander Knox], outside Esmond's affluent home, but the doctor overpowers Clemmons. Arriving home from Paris, Esmond's wife Glenda Esmond, [Alexis Smith], is taken aback to discover Clemmons staying in their home as a household guest. To avoid being turned over to the police, Clemmons has agreed to stay, acting as a human guinea-pig subject to Esmond's psychoanalysis, which aims to release him from his criminal recidivism. Glenda has reservations about Clemmons and behaves in a cold, aloof manner towards him.
Clemmons undergoes regular analysis with Esmond, who is determined to get to the root of his criminality. In between these sessions, he goes riding with Glenda. Although at first indifferent to him, she soon finds herself growing attracted to him. With a fellow criminal in town, Clemmons leaves the house one night and steals some jewellery. A police inspector, [Hugh Griffith], later interviews him about the crime, but he denies having committed it. After a while, Clemmons takes Glenda to the Metro, a hipster nightclub in Soho, where her conflicted attraction to him deepens. The next day, Glenda admonishes Clemmons for his violent behaviour towards a house-maid, Sally Foster, [Patricia McCarron], but their argument ends with a passionate clinch which indicates the beginning of an affair between them.
Initially oblivious, Esmond eventually finds his wife in a compromising position with Clemmons. Glenda's ambivalent feelings plague her. Back at the Metro club with Clemmons, he tosses a careless remark Glenda's way and this overwhelms her. As they begin their journey home, she is driving recklessly and out of control. A police car pursues them, but they manage to escape it.
Sally Foster's fiancé visits Esmond to complain about the abuse she has endured from Clemmons and threatens to tell the police. No charges are pressed and Clemmons learns that this is due to Esmond buying the man off with £100. He reacts by carrying out another robbery. When questioned by the police, Esmond ends up lying on behalf of Clemmons. A cunning ploy, this results in Clemmons pouring out a dramatic account of his tyrannical father, whom he deeply despised. As a boy, Clemmons stole and his father consequently turned him in to the authorities. Frank vowed revenge on his father when he was released, but was then given a beating. His father died shortly thereafter, and his mother blamed him. Esmond concludes that since his father's death, Clemmons has had to provide his own punishment for the rest of his life.
Esmond soon begins acting like a father figure towards Clemmons. The two enjoy carefree activities together; Glenda grows intensely jealous. She asks Clemmons to elope with her. However, with Esmond's psychiatric experiment over and his patterns of behaviour understood, Clemmons decides to turn himself in to the police. Glenda hysterically rushes to Esmond, claiming that Clemmons has assaulted her. Esmond goes upstairs with a gun. A shot is heard and Esmond returns claiming that he has killed Clemmons. Glenda is heartbroken and ends up declaring her love for Clemmons. She then finds out that although there was a gunshot, Clemmons had not been in the room, he had already left. She goes after him in her car and comes upon him walking. She tells him to get into the car. He does and she subsequently drives off high speed. Highly distressed, Glenda swerves to avoid a lorry, but crashes the car. Clemmons survives, but Glenda dies in the wreckage.
