When DS Steve Arnott joins the anti-corruption unit, he learns that the target of his first investigation is the well-respected DCI Tony Gates. Though Gates’ professional conduct appears above reproach, his private life may provide his enemies the evidence they seek.

Having been duped into covering up Jackie’s hit and run, Gates is desperate to sever ties with her. But as Arnott and Fleming close in on his secret, Gates is plunged into an even more dangerous situation.

Arnott is convinced Gates played a part in Jackie’s disappearance. With Fleming’s help, the anticorruption team try to trap Gates into revealing his involvement in covering up Jackie’s crimes.

Desperate for proof of Gates’s corruption, Fleming pushes her relationship with him to the limit. But her behavior causes Morton to question if she might not be such a loyal colleague after all.

Tensions erupt between Hasting, Arnott and Fleming as they decide on the next steps in the case. Increasingly desperate and unwilling to await his fate, Gates goes on the run, ready to play one last card.

A brutal ambush on a police convoy leaves three officers dead and a protected witness fighting for his life. AC-12 turns its spotlight on Lindsay Denton, the officer who led the convoy but escaped unscathed.