Valerie Craven, the daughter of a local building magnate, has been missing for six months, and an otherwise-idle Morse is assigned the case. When a letter with a London postmark arrives, purportedly from the missing girl, initial inquiries take Morse and Lewis in the direction of her former boyfriend John Maguire. Guesswork from Morse surprisingly strikes home, and it is established that the girl is, or was, pregnant.
The headstrong headmaster of Valerie's school, Donald Phillipson, and a former teacher, David Acum, become suspects. When the deputy head, Cheryl Baines, is found dead at her home, Lewis lashes out at Morse for his previous insistence that the case involved murder, now confirmed. Strange calls him to task for bending the rules, particularly for forging a second note from Valerie to the police.
A neighbour of Baines's reports seeing a woman go into Baines home on the evening of her death, and Lewis realises it was Phillipson's wife Sheila. She comes in for questioning and insists that all she encountered at the home was Baines's body at the bottom of the stairs. She tells Morse and Lewis that, as she left, she saw Acum waiting in a car around the corner.
Acum is brought in, but after a brief questioning, Morse seems content to let him go. He insists on driving Acum back to Reading, and when Acum claims his wife is not home, Morse gladly accepts the invitation to wait for her. Once inside Morse calls out for Valerie, and she emerges from upstairs, Morse having realised that he had already met Valerie when he had called at Acum's house earlier on but did not recognise her, as she was wearing a face pack at the time.
Morse brings Valerie home, and she finds her mother in a discussion with Phillipson and his wife. Phillipson admits that he and Grace Craven were having an affair and claims they were together the night Baines died. Despite previously confirming that to Morse, Grace now insists he was lying and that the affair ended months ago. Valerie explain that she had visited Baines on the evening in question and that she saw Phillipson arrive as she was leaving. After claiming that both Grace and Valerie were lying, Phillipson finally admits to a struggle with Baines that led to her accidental death, and is taken into police custody.