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.

Brian Lane knows his days at UCOS are numbered. Meanwhile the team investigate a 16-year-old murder of a young mum by her criminal husband that could be a miscarriage of justice. However Gerry Standing feels very strongly that the conviction should not be called into question.

As Lane comes to terms with life after UCOS, Esther asks him to help one of her book club friends find her missing brother - but will his nose for the truth get him into further trouble? And how will his former colleagues cope without their most dogged sleuth?

When blood-stained clothes belonging to Simon Belgrade, missing for five years, are uncovered in a London forest, UCOS find themselves trying to solve the conundrum of what seems to have been the perfect vanishing act. Standing finally faces up to a childhood fear, while Griffin is a comfort to Pullman as she passes a milestone and McAndrew makes a big decision about his son.

When the team investigate the death of an MP's researcher and find themselves in hot pursuit of a mysterious Bosnian woman, Strickland introduces Pullman to a war crimes prosecutor who proves to be more than helpful to their case.

Pullman seizes the chance to solve a murder that she believes is linked to her first ever case, and the team start putting together a jigsaw puzzle of suspects from a photograph taken in a London park in the 90s. Eventually faced with increasing pressure to hand over the investigation, Sandra makes a huge decision, which is going to change UCOS forever.

With Sandra having left, the team wait to meet their new boss, and Sasha Miller wastes no time fitting in and getting on with her first Ucos case. It features the murder of an Italian immigrant on a west London allotment 25 years ago.

Miller is faced with a dilemma when the investigation of a corrupt senior officer calls into question the conviction of Edward Monroe - who she believes to be guilty of murdering her old work partner. Pre-empting an appeal, UCOS must carry out preliminary investigative work for Strickland.