All the episodes from the first four series of the British crime drama starring Martin Compston and Vicky McClure. In the first series, Detective Sergeant Steve Arnott (Compston) is transferred to AC-12, a unit assigned to investigate corruption within the police force. Working with Detective Constable Kate Fleming (McClure), Arnott quickly finds himself immersed in a case involving Detective Chief Inspector Tony Gates (Lennie James). In the second series, an ambush of a police convoy sparks alarm when three officers are killed and a witness in protective custody is seriously injured. With only one survivor, Detective Inspector Lindsay Denton (Keeley Hawes), department AC-12 is brought in to solve the case. In the third series, Sergeant Danny Waldron (Daniel Mays) and his team are assigned the case of a suspected gang member who has been arrested on suspicion of plotting a gangland murder. When Waldron shoots the suspect in cold blood and stages the scene to look like self-defence, DI Denton brings in Waldron for questioning. In the fourth series, AC-12 Superintendent Hastings (Adrian Dunbar) places DS Fleming undercover to investigate a possible miscarriage of justice in a murder investigation. Head of the case DCI Roz Huntley (Thandie Newton) thinks she's finally found her suspect after months of searching for a serial killer, however, those at AC-12 are led to believe she may be concealing valuable evidence. As new revelations begin to emerge, will they be able to identify the real perpetrator?