Watch Line of Duty Online at Couchtuner. Following one multi-stranded investigation over five hours, Line Of Duty sees Detective Sergeant Steve Arnott (Martin Compston) transferred to AC-12, a fictional anti-corruption unit, after a mistaken shooting during a counter-terrorist operation. Alongside Detective Constable Kate Fleming (Vicky McClure), they are assigned to lead an investigation into the alleged corruption by a popular and successful officer, Detective Chief Inspector Tony Gates (Lennie James). While Gates cleverly manipulates his unit’s figures, DS Arnott questions whether Gates’s being made a scapegoat for a culture of institutionalised spin, or is guilty of darker corruption?