The plot centres on a serial killer in contemporary London and the ex-RAF serviceman he implicates. In a very early scene there is dialogue that mentions two actual London serial murder cases: the Christie murders in the early 1950s, and the Jack the Ripper murders in 1888. Barry Foster has said that, in order to prepare for his role, he was asked by Hitchcock to study two books about Neville Heath, an English serial killer who would often pass himself off as an officer in the RAF.