An unknown menace stalks Oxford, appearing at first to be just another deranged lunatic, killing at random. (Morse meets Constable Strange for the first time in this episode) Following the shooting of a vicar, Morse is reduced to general duties for dismissing a beautiful, but mentally unstable girl as a suspect and must continue his investigations alone despite warnings of possible dismissal from his superiors. Chief Superintendent Bright, the new commanding officer of the Oxford City Police, unimpressed with Morse's zeal, protests to Thursday that the bagman's position is a Detective Sergeant's job and the young constable is too inexperienced. The sudden death of a secretarial student (Margaret Bell) and the shooting of a doctor appear unconnected despite Morse's theories. Endeavour's superior, Detective Inspector Fred Thursday, recognising the young constable is a detective he can trust, takes him under his wing and is determined to break the case and, with Morse's help, bring it to a successful conclusion.

The murder of a 15-year-old schoolgirl and the apparent suicide of her boyfriend lead an investigation by the Oxford City Police to the discovery of sex parties where under-age girls are procured for politicians, businessmen, Oxford dons, and policemen, which in particular make the sifting of evidence very difficult.