Unfaithful housewife Evelyn Balfour and elderly botanist Grace Madison are both murdered in the style of operatic heroines with the words of the characters' final arias being left at the scenes of crime. Psychiatrist Daniel Cronyn is brought in and informs the police that they are dealing with a serial killer. He believes it might be Keith Miller, a former patient of his who was a musical prodigy. A third murder at the killer's apparent hide-out reveals a press cutting that suggests that Morse is one of several victims that have been lined up. Ultimately Morse and Thursday find themselves unwittingly re-enacting the second act of 'Tosca' on a roof-top as they bring the killer to justice.