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Steve Arnott 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; as Arnott and Fleming close in on his secret, Gates is plunged into a more dangerous situation.

Gates comes round in Jackie's house. Her blood stains the carpet, but there's no sign of her body. As Arnott races to the house, Gates must act fast. Arnott is convinced Gates played a part in Jackie's disappearance. With Fleming's help, the anticorruption team tries to trap Gates into revealing his involvement in covering up Jackie's crimes. Desperate to get his man, Arnott oversteps the boundaries, risking the entire anticorruption operation - and his own career. Meanwhile Gates wrestles with the nagging doubt that Jackie may still be alive.

With Arnott off the case, Fleming is determined to save the anticorruption operation single-handed. Desperate for proof of Gates' involvement in Jackie's crimes, Fleming pushes her relationship with him to the limit. She must test how far she'll go to gain his trust. However, her behaviour causes Morton to question if she harbours an ulterior motive. If Fleming makes the tiniest slip-up, Morton will blow her cover.

With Gates on the run, the anticorruption team comes under pressure to close the case once and for all. Tensions flare between Arnott, Fleming and Hastings, especially when Arnott develops suspicions about just how intimate Fleming was prepared to get with Gates. Meanwhile, Gates still believes he can save his career and marriage, if only he can prove his innocence. Arnott is forced to make some tough decisions about where his loyalties lie, while Fleming and Hastings go all out to catch Gates, leading to a shattering conclusion.

The series opens with the violent ambush of a police convoy in which three officers are killed and a protected witness seriously injured. When evidence suggests that a police source may have leaked the whereabouts of the convoy to criminal parties, DCC Mike Dryden (Mark Bonnar) takes personal charge, assigning anti-corruption unit AC12 to the case. With DS Kate Fleming (Vicky McClure) reluctant to join an investigation she has personal links to (she trained with one of the ambush victims) DS Arnott (Martin Compston) and Superintendent Hastings (Adrian Dunbar) are joined by new recruit DC Georgia Trotman (Jessica Raine). Initially AC12's most valuable witness, the suspicion soon arises that the sole surviving police officer, DI Lindsay Denton (Keeley Hawes) could be their prime suspect. Did she lead the convoy straight into the fatal ambush?