Major screws up during an investigation and Carver wonders whether to turn him in. But when he learns of the consequences, he decides to cover for him.
After he and Carver are given a new assignment by Dennett, a suspicious Major prefers to stake out Kelly's work as he has a hunch she's been seeing someone else.
Major, Carver and Kelly have gone their separate ways. Things are looking up for the AI-enhanced Major, whilst Carver is gunning for a promotion and is knee deep in a new case.
Carver and Major receive new information on the Chelsea Bridge Case – a £50 million robbery committed twenty years ago by a gang who seemed to vanish into thin air.
DCS Dennett officially reopens the Chelsea Bridge case, reuniting Major with his absent father, Clifford, who failed to solve the case 20 years earlier. As Major's AI becomes increasingly unstable, Dr. Parfit and Carver clash over the solution.
Carver deals with an unstable Major, and Dennett calls in a favour from Millikan Biotech. When the detective inspectors are assigned to a witness protection case with some familiar faces, Carver learns shocking truths and confronts Major.
When Carver and Kelly find a collapsed Major with just three hours left to live, they try to get him to accept the truth. Major is determined to solve the Chelsea Bridge case and exonerate his dad before his AI shuts down.
Major and Carver reunite, but tensions remain high. When a police officer is murdered, Carver becomes hell-bent on solving the case, Major has an epiphany and resolves to become a better man, and both detective inspectors are out to prove themselves.
With a serial killer at large, Major and Carver call on the help of forensic psychologist Professor McAllister. Parfit worries about Major's AI updates.
Chalmers, the former police commissioner who has plagued Major since the day he was reanimated, faces trial. With Major busy testifying, Carver is running out of time to decipher the latest clue and save the killer's next target.