Following one multi-stranded investigation over five hours, Line of Duty sees Detective Sergeant Steve Arnott transferred to AC-12, an anti-corruption unit, after a mistaken shooting during a counter-terrorist operation. Alongside Detective Constable Kate Fleming , they are assigned to lead an investigation into the alleged corruption by a popular and successful officer, Detective Chief Inspector Tony Gates. While Gates cleverly manipulates his unit's figures, DS Arnott questions whether Gates is being made a scapegoat for a culture of institutionalized spin, or is guilty of darker corruption.