Tim Brett, David Hemmings, is a former drug addict who has written a book about his experiences, which has been published. He has been clean for about a year, and visits his aunt, a wealthy philanthropist, in an Italian coastal hotel. She expresses interest in helping some of Brett's former acquaintances.
Soon after, on a tour of Pompeii, visitors discover the body of his aunt, who has been strangled. At the elaborate funeral Brett meets Signor Bardoni, Adolfo Celi, the hotel owner who organized the service. He says it is ironic that his aunt has been killed by a criminal when she had spent her life <em>“helping criminals”</em>.
> helping criminals
A card on a wreath at the funeral says it is from "The Stepping Stones," which Bardoni attempts to conceal.
Brett starts a relationship with Juliet Bristow, Gayle Hunnicutt, one of the women who found his aunt's body, and they are soon engaged. Six months after his aunt's murder and back in England, Brett grows dissatisfied with the progress the police are making in the case, and he begins to question some of his aunt's acquaintances. He then begins to receive warnings from unknown persons to stop his inquiries.
On the train he meets an elderly woman. She hands him a note of supposed comfort, asking him to read it at home. The note proves to have been typed on his own typewriter, and it warnings him to leave matters to the police. He also finds an ominous laugh recorded on his own tape recorder, and a cigarette end in his toilet, indicating that someone has been in his flat.
Sgt. Matthews, Derek Newark, a police sergeant, informs him that the woman on the train has lodged a complaint against Brett. Sgt. Matthews takes Brett's information and the note, but after the woman is found dead, Brett discovers that there is no sergeant by that name working at the police station.
He makes contact with a secret government agency which tells him that they are after the people who are threatening him. They explain that The Stepping Stones, which was originally set up by his aunt to help criminals become good citizens, has become an organisation blackmailing those who become successful. Brett is later assaulted on the streets at night by two men who leave him lying on the ground with a hypodermic needle. He throws the needle away down a gutter. Later, he receives a phone call implying Juliet will be harmed at their wedding, and his mental state deteriorates seriously.
At their wedding, Brett becomes paranoid about the attendees and starts to hallucinate. He drags Juliet out of the church before they have been married and flees. He continues to hallucinate on the train. In the closing scene, Juliet pushes the unstable Brett in a wheelchair.
