Season 1 of "Line of Duty" consists of 6 episodes and first aired from June 26 to July 24, 2012, on BBC Two. The series introduces viewers to the Anti-Corruption Command (ACC) within the Police Service of Northern Ireland (PSNI), focusing on the unit's efforts to tackle corruption within the police force.

The story revolves around Detective Inspector Steve Arnott (Martin McDonagh), a honest and dedicated officer who works for the Police Ombudsman's Office (POLIT, the internal affairs division). When a high-ranking police officer, Chief Superintendent Ted Crawford (Douglas Henshall), is gunned down in a botched shootout, Arnott is tasked with leading an investigation into the incident. As the case unfolds, Arnott teams up with Detective Constable Gail McCormick (Sharon Horgan) and together they uncover a vast web of corruption within the PSNI.

Gates, the leader of the TO-20 squad, is "Officer of the Year" with an impeccable arrest record. However, AC-12 suspects him of ""—a process of inflating crime statistics by adding multiple charges to a single defendant. What begins as a search for minor administrative corruption quickly spirals into a dark web of murder, money laundering, and organized crime. Cast and Key Characters

| Character | Role | Key Trait | |-----------|------|------------| | Supt. Ted Hastings | Head of AAC | “Hastings’ Rule” – fiercely anti-corruption | | DS Steve Arnott | Investigator | Rigidly moral, tech-savvy | | DS Kate Fleming | Undercover officer | Loyal to Hastings, but conflicted | | PC Jackie Laverty | Witness/suspect | Local businesswoman, linked to Gates |

The series has also launched the careers of several actors, most notably Gerard Butler and Stephen Graham, who appeared in later seasons.