Clinical psychologist Dr Tony Hill and Detective Chief Inspector Carol Jordan face the toughest challenge of their careers when they are confronted with an 'impossible' series of murders. Back in Bradfield after her traumatic experiences in Berlin, Carol is surprised to find Tony has followed her there to take up a post in the local secure mental hospital. When a prostitute is murdered in a particularly grotesque and stomach-churning way, she turns to him for help. Bizarrely, this killing matches in every detail a series of murders that took place a couple of years previously. A series of murders for which Derek Tyler was tried, convicted and sentenced. There has never been any doubt about his guilt. But now the ghost of his crimes has risen again. A second prostitute murder soon follows and the team are struggling. While Tony tries to crack Tyler, the police decide to mount an undercover operation that goes horribly wrong.