British crime drama starring Martin Compston, Vicky McClure and Lennie James. As punishment for a previous failure, 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), he quickly finds himself immersed in a case involving Detective Chief Inspector Tony Gates (James). With Gates a slippery and resourceful adversary, the pair will have to keep their wits about them if they are to crack the case.