Tim Brett (David Hemmings) is a former drug addict who has written a book about his experience and has been published. He has been clean for about a year. He had recently become acquainted with his aunt (Flora Robson), a philanthropist who expresses interest in helping some of Tim's former acquaintances. She is found murdered soon after. Tim starts a relationship with Juliet (Gayle Hunnicutt), the woman who found his aunt's body, and they are soon engaged. Dissatisfied with the progress that the police are making in his aunt's murder case, he begins to ask questions of some of his aunt's acquaintances. He then begins to receive warnings from unknown persons to stop his inquiries. He meets an elderly woman on the train. She hands him a note of supposed comfort, asking him to read it at home. The note turns out to be a warning about leaving matters to the police, apparently typed on his own typewriter. There's also an ominous laugh recorded on Tim's own tape recorder, indicating that someone had been in his apartment. Tim is then visited by a police sergeant, Sgt. Matthews, who informs him that the woman on the train had lodged a complaint against Tim. Sgt. Matthews takes Tim's information but after the woman is also killed, Tim finds out that there is no sergeant by that name working at the police station. Tim is later assaulted on the streets at night by two men who leave him lying on the ground with a hypodermic needle. Tim throws the needle away down a gutter. He makes contact with a secret government agency which tells him that they are after the people who are threatening him, but all is - again - not what it seems to be. As the situation continues, Tim and Juliet's wedding fast approaches.