S2, E1: 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?