Thirty years ago, Strickland and a group of his associates, including Stephen Fisher, were involved in a covert operation on behalf of MI5. Now, someone is trying to kill all the members of that group and Strickland turns to the UCOS team for help.

Lane's future at UCOS is threatened by a death in custody which emerges from his past. Meanwhile, the discovery of a mysterious gun in the Thames re-opens the unsolved 1998 murder of a playboy shipping heir and leads the team to the Rock of Gibraltar.

The team unlock over three decades of secrets and lies on the Rock of Gibraltar and find the link which solves the murder of a playboy shipping heir from '98 and a young local boy from '82. With both cases solved, they return to England where Lane faces a disciplinary hearing, dragging up a death in custody from his past which threatens his career in UCOS.

UCOS want to talk to a Turkish girl who is found working illegally in Dalston, regarding a unsolved murder from 2009. The landlord of a pub, Richard Gibson, died in a fire and the case had remained unresolved. The team meet the landlady of the pub, Joanna Gibson, who is also Richard's widow. His best friend, David is also someone of interest, especially when the team find out the French champagne he is selling, is actually sparkling wine produced in Britain. Sasha enters the team into a 5-a-side football competition.

While investigating the murder of 55 year-old interpreter Agnes Bradley, the UCOS team get a partial DNA match from a teenage boy who once threw a brick onto a motorway as a dare. They interview the boy to see if one of his male relations could be the murderer, only to discover that his mother was raped and they could be looking for a rapist-turned-murderer. Agnes was a chess enthusiast so Gerry Standing and Steve McAndrew interview members of her local club while Sasha Miller and Strickland investigate leads from Agnes’s working life, where they meet a Chilean diplomat with a possible axe to grind. Steve McAndrew’s elderly father is dying, so he brings him to a London hospice for his last days.