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).

[6] 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).

[10] 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.

[16] 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.

[17] 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.

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.

[46] 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.

[50] 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"[51] and that its challenging plot is rewardingly rare for a series in the summer timeslot.