When a counterterrorism raid goes wrong, DS Steve Arnott refuses to join the cover-up. Cut off by colleagues, he transfers to an anti-corruption unit that's investigating one of the force's most respected officers, DCI Tony Gates. A devoted family man, Gates has a secret he wants to keep that way, but one lie may expose him to the greatest risk of his life.

Having been duped into covering up a crime committed by Jackie, his mistress, Gates is desperate to conceal any evidence. Suspicions are soon aroused; and a new-hire on the inside, DC Kate Fleming, begins working secretly with Arnott. As the net closes on Gates, he's forced to choose between his marriage and his career, and finds himself plunged into an even more dangerous situation than he thought.

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 behaviour causes Morton to question if she might not be such a loyal colleague after all.

With Gates on the run, the anticorruption team come under pressure to close the case once and for all. Arnott is forced to make some tough decisions about where his loyalties lie. Fleming and Hastings go all out to catch Gates, leading to a shattering conclusion.

A brutal ambush on a police convoy leaves three officers dead and a protected witness fighting for his life. AC-12 - Ted, Steve and new recruit Georgia - turn their spotlight on Lindsay the officer who led the convoy but escaped unscathed.