Boyd is informed he will be moved out of the Cold Case Unit, in the penultimate episode of the crime drama. As his last case, he chooses to re-investigate the first case he worked on: the disappearance of 16 homeless teenage boys between 1979 and 1982.
The boys were taken at precise three-month intervals, suggesting their disappearance was the work of a sociopath. However, the team have no new leads. Grace puts it to Boyd that his choice to take this as his last case, so close to his heart yet almost certainly unsolvable, is some desperate form of displacement: he still hasn't properly grieved for his dead son, Luke.
When the team learn of the road death of a teenage boy in 1982 on the M11, they close in on the abductor's horrific lair – an underground torture chamber. Eve sets to work and discovers the dismembered corpses of all the missing boys and the team's first lead: the gun used to execute them was also used in the killing of Stanley Heath, a police officer, in 1959.
Boyd interrogates the officer's son, Jason, and learns that his father carried a terrible secret – the murder of a man in custody. Of particular interest to the team, the man in question was father to the current Assistant Chief Commissioner, Tony Nicholson. And Nicholson, as a young PC many years ago, collared a man attempting to abduct a homeless boy in Soho ... but let him go.
A guilt-ridden Sarah decides to follow Nicholson to learn more of his interest in the case, in a desperate attempt to atone for her role in Boyd's sacking. With Boyd more isolated and reckless than ever, the team uncovers a staggering secret from the past, which finds them playing a chess game for their lives...