After the empty and abandoned car of banker Donald Rees is discovered threee years after he went missing, Boyd decides to take on the case in an effort help the man's family find some closure with their lives.
Boyd starts to think he has come to know the missing man and he believes that that Donald Rees is not a murderer. He ends up getting the team to investigate harder at all the different possibilities.
After a woman is discovered stabbed in the eyes and incinerated in her car she is identified as Claire Somers, a neglected child who was taken into care, then later kidnapped in 1986 at the age of seven. Robert Fenchurch had his eyes stabbed out on the night Claire was kidnapped as her abductor fled the scene. The team soon find out that Claire had snatched 15-year-old Abigail Harding, who was found locked in a hotel room. Abigail tells the team what she can remember. The team end up finding a doctor who had been treating Claire, and are surprised to find out it is Teresa Harding, the elder sister of the kidnapped Abigail. The team then finally manage to get a lead on the case.
The team work out that Teresa was running from someone or something in her room and would rather have taken her own life than face up to it. Peter Broading's psych report ends up giving an important clue. Boyd manages to break Broading and he ends up admitting that he was part of Fenchurch's regime. He then reveals that David Drew, a boy of just nine years old was in charge of the violence that went on. Trish ends up giving Sarah a new lead in the case and the team face a race against the clock to solve the case before more blood is shed.
The team are asked to go to a chamber of a little-known subterranean river. The body of a Falklands' veteran-turned-peace activist called Piers Kennedy who has been missing since 1983 has been found directly under the building of the Ministry of Defence.
Grace ends up admitting to the team the truth about her relationship with Murray. She insists though that Murray was actually relieved that the PR stunt by CANW didn't make the news. After looking at the video footage agan, the team see Piers touching the warhead on purpose as he wanted to be identified and caught. They talk to Bonnie, but she insists that she was not the female activist and has no idea what Piers was planning to do. Boyd wants Grace to get information out of Murray despite both Sarah and Spence warning of the dangers.
Karl Barclay a medical student who vanished six years ago comes to the attention of the team when his DNA matches that of a body also discovered six years ago in Hales reservoir. Eve finds signs of torture on the body and Boyd starts to become suspicious that there has been a cover up in the past. Boyd soon discovers that there had been tension in his family that exploded after Karl's evangelical Christian mother, Lisbetta, found out about his conversion to Islam. After becoming unhinged Lisbetta attacks a member of the public, thinking that he killed her son. Lisbetta end up admitting to Gideon that she had to stop her son.
The team think of Karl as a would-be bomber who after backing out from a planned attack was killed by other extremists. Forensics end up indicating that Karl fell to his death from an airplane which points the finger at a state murder and a cover up.
Boyd learns that he is to be moved out of the Cold Case Unit. He chooses for his last case to re-investigate the first case he worked on which was the disappearance of 16 homeless teenage boys between 1979 and 1982.
Boyd goes back to the torture chamber and discovers that Nicholson is already there and admitting that he did indeed let the killer go all those years ago. Nicholson believes that Jason Heath is the responsible but Grace is not as convinced. Sarah ends up going missing as the investigation continues and after her car is found her friends grow more concerned. Boyd is instructed by Nicholson to remain focused on the M11 murders and he will investigate Sarah's disappearance. The team's suspicions about Nicholson increase.