Steve Arnott is transferred to a police anti-corruption unit, and soon discovers his target is the city's top detective, Tony Gates.

Having been duped into covering up Jackie's crime, Gates is desperate to sever ties with her. However, as Arnott and Fleming close in on his secret, Gates is plunged into an even more dangerous situation than he first thought.

Arnott is convinced Gates played a part in Jackie's disappearance. With Fleming's help, the anti-corruption team tries to trick Gates into revealing his involvement in the cover-up.

Determined to find proof of Gates's corruption, Fleming pushes her relationship with him to the limit. However, her behaviour causes Morton to question if she might not be such a loyal colleague after all.

Gates comes back to rescue Arnott; Kate's cover is blown but she finds evidence that a body was stored in the warehouse.

A brutal ambush on a police convoy leaves three officers dead and a protected witness fighting for his life. AC-12 - Ted Hastings (Adrian Dunbar) Steve Arnott (Martin Compston) and new recruit Georgia Trotman (Jessica Raine) - turn their spotlight on Lindsay Denton (Keeley Hawes) the officer who led the convoy but escaped unscathed.