The series follows the actions of the fictional Anti-Corruption Unit 12. AC-12 is led by Superintendent Ted Hastings (Adrian Dunbar), who recruits DS Steve Arnott (Martin Compston) and is backed by undercover officer DC Kate Fleming (Vicky McClure), as they lead an investigation into the corrupt actions of DCI Tony Gates (Lennie James). Gates leads Tactical Operations Unit 20 (TO-20), which includes DS Matthew "Dot" Cottan (Craig Parkinson) and DC Nigel Morton (Neil Morrissey). Supporting characters include Jackie Laverty (Gina McKee), Jools Gates (Kate Ashfield), Chief Superintendent Derek Hilton (Paul Higgins), Chief Inspector Philip Osborne (Owen Teale), and Tommy Hunter (Brian McCardie).

The series was created and written by Jed Mercurio, who also serves as a producer. Filming took place in Birmingham in late 2011. Five episodes were directed by David Caffrey and Douglas Mackinnon with cinematographer Ruairi O'Brien. Despite being censured by Ofcom, the series received mostly positive reviews and was nominated for multiple awards, leading to the commission of a second series.

• Lennie James as DCI Tony Gates
• Martin Compston as DS Steve Arnott
• Vicky McClure as DC Kate Fleming

Series creator and producer Jed Mercurio stated that, when creating Line of Duty, he wanted to create something different than other already existing programmes and viewed the series as a "revisionist commentary on 21st-century policing". The series is produced by World Productions. Mercurio originally pitched the series to BBC One who turned him down, believing the series was too "problematic for mainstream audiences," and directed him to BBC Two.

Leading the main cast are Lennie James, playing DCI Tony Gates, as well as Martin Compston and Vicky McClure, who portrayed two officers in the fictional Anti-Corruption Unit 12 (AC-12). James was offered the role without an audition and returned to the UK from Los Angeles for filming. He described his character as morally grey, comparing the concept to the people involved in the Syrian civil war. Compston ultimately played the role of Steve Arnott, having to assume an English accent for the series. While auditioning, McClure read parts of the script with multiple people who were recalled for the role of Arnott and performed extensive research into the police force before portraying of DC Kate Flemming. Gina McKee, Neil Morrissey, Adrian Dunbar, Kate Ashfield, and Craig Parkinson all held supporting roles. Parkinson originally auditioned for the role of Arnott.

Principal photography began in August 2011 with an anticipated airing in early 2012. Five hour-long episodes were directed by David Caffrey and Douglas Mackinnon for BBC Two. Ruairi O'Brien worked as the cinematographer on all five episodes which were filmed over 11 weeks in Birmingham and utilized two cameras. Mercurio later said that, despite filming in Birmingham, the setting was later changed to the Midlands at the advice of the BBC. Following the series broadcast, Ofcom held an industry meeting and censured the BBC for not exercising a "duty of care" for Gregory Piper, a 13-year-old actor who was "exposed to sexually explicit language" and adult situations during production. The BBC responded saying they had acquired permission from Piper's parents to which Ofcom cited their policy of requiring a counsellor or psychologist on set to evaluate the possibility of emotional effects. A second series was commissioned by the BBC on 25 July 2012.

On BBC Two the series was first broadcast beginning on 26 June 2012 and concluding five weeks later on 24 July. It was also simulcast on BBC HD. In the United States Line of Duty was released on Hulu weekly between 21 August and 18 September 2012 and was later added to Acorn TV in 2018 and BritBox by 2021. It was also broadcast on the basic cable television network AMC beginning on 4 April 2020. In Canada and Australia the series is available on Netflix. The series was later re-broadcast on BBC One beginning 3 August 2020 due to COVID-19 filming shutdowns delaying production of the sixth season.

By the time the first series had concluded airing, Line of Duty was the fifth highest-rated series to air on BBC Two since 2002. The series maintained a 15.7% share in consolidated data.

On Rotten Tomatoes, a review aggregator website, the series holds a 100% rating based on five reviews. Metacritic, which uses a weighted average, gives it an 83/100 indicating "critical acclaim". Writing for The A.V. Club, Sonia Saraiya notes that the story being told from a journalistic standpoint with minimal backstory, and the use of a small main cast, make for a unique series. Reviewing the series as a whole, The Guardian's Rhik Samadder opines that it's difficult to follow the pacing, stating, "One minute it's Men Behaving Badly, the next someone with Down's syndrome is being slapped in the face."

Sarah Hughes, also with The Guardian, wrote individual episode reviews and says after episode one that it's hard to oppose the character of Tony Gates with the captivating acting from Lennie James. Following episode 3 she writes that Line of Duty performs better in its slower paced episodes compared to action-packed ones. Reviewing the final episode of the series, Hughes reaches the conclusion that it would have been better with more episodes believing some story elements were rushed.

CultBox also published individual episode reviews from Rob Smedley, who stated, "Jed Mercurio has already crafted a police drama that feels as claustrophobic and threatening as a night sharing a cell with a convict and his dog" and that its challenging plot is rewardingly rare for a series in the summer timeslot. Ceri Radford from The Daily Telegraph believes that the difference between the characters' personal and professional lives were balanced properly.