Morse and Lewis are called to St Oswald's where the churchwarden, Harry Josephs, is reported to have been stabbed by a local vagrant following a service. While the hunt for the vagrant begins, the vicar, Lionel Pawlen, and his congregation are all eyed with suspicion. Max the pathologist informs Morse that the dead man had a lethal amount of morphine in his system when he was stabbed, suggesting that two people tried to kill him.
Tipped off by another vagrant that the suspect may be the vicar's brother, Morse invites Pawlen in for questioning, but while Morse and Lewis are waiting for him, Pawlen throws himself off the church tower. Meanwhile, Morse takes a romantic interest in the church caretaker, Ruth, who seems amenable but is somewhat evasive to his attention. Several scenes make it clear that she is having an affair with an as-yet unidentified man.
Over the next few days, Paul Morris, the organist, is found murdered, and Brenda Josephs, with whom Morris had been having an affair, and Morris's son are all found dead. Morse returns to the church and hides in the confessional. When Ruth arrives, she is confronted by her lover, who attempts to strangle her. Morse intervenes and after an awkward struggle, follows the man up the tower stairs, despite his fear of heights. The two men fight at the top of the tower, until Lewis arrives and hits the attacker over the head with a candlestick. He staggers away and falls over the edge to his death.
As they take away the body, Morse explains that the killer is Harry Josephs, who was thought to have been the original victim. Lionel Pawlen and his congregants conspired in the original murder of Lionel's brother Simon, the vagrant. Simon had been left out of a great inheritance from an aunt owing to his wayward lifestyle and had grown vindictive towards Lionel, spreading rumours about the vicar's behaviour towards choir boys that had forced him out of his previous parish.
The original murder was an elaborate way of getting rid of Simon. Lionel used an invented church service, The Feast of the Conversion of St Augustine, and a conspiracy of his congregants, with whom he would share his inherited wealth, to execute the plan. The congregants agreed falsely to identify the body as Harry Josephs, who used this cover to exact revenge on his wife, Brenda, her lover, Paul Morris, and Morris's son. Lionel's death was suicide, spurred by the guilt of his brother's murder and suspecting that his deceit would soon be uncovered by Morse.