Line of Duty follows Detective Seargent Steven "Steve" Arnott (Martin Compston), a former authorized firearms officer who led an operation to catch an Islamic terrorist intending to detonate a bomb, but the strike team mistakenly entered the wrong apartment and shot dead an unarmed innocent man. In the subsequent investigation, Arnott refused to cover up negligence in the planning and execution of the raid and unable to work in his unit anymore, Arnott receives a transfer to AC-12, a unit tasked with uncovering police corruption. Arnott is partnered with Detective Constable Kathrine "Kate" Fleming (Vicky McClure), a highly commended undercover officer with a keen investigative instinct. They work under the supervision of Superintendent Edward "Ted" Hastings (Adrian Dunbar), who is one of several Senior Investigating Officers responsible for uncovering corruption within the police force. Throughout the series, AC-12 investigate seemingly disparate cases involving seemingly corrupt police officers, with each series focusing on a different corrupt officer and AC-12 soon realize the pervasive nature of corruption and the police's deep-rooted links to an organized crime group with ties to drug running, prostitution, child exploitation and people trafficking.