Morse and Lewis investigate the disappearance of teenager Valerie Craven who vanished some six months previously while walking home from her exclusive girls school. Morse is the third detective assigned to the case and is convinced that the girl is by now dead. However, the girl's parents receive a note, purportedly from Valerie. Suspicion falls on a number of people, including fellow students, men named in her diary and some of the teachers at her school. When one of the teachers is killed, the missing persons enquiry becomes a murder investigation.