Supporting characters include underboss Lisa McQueen (Rochenda Sandall) and the special counsel to the police and crime commissioner Gill Biggeloe (Polly Walker).

[6] The special was promoted as a "deleted scene" from the fifth series and starred Compston, McClure, and Dunbar along with Jason Isaacs as DC Taylor and Lee Mack as David Rickman.

[10] The plot includes elements of the main programme, but the events of the supplemental episode do not fall within the continuity of Line of Duty.

[24] Radio Times drama editor Eleanor Bley Griffiths wrote: "With epic interrogation scenes, surprise betrayals, grisly murders, untold secrets, massive twists... "[25] Pat Stacey of The Irish Independent wrote: "Episode by episode, the plot is thickening, like soup that's been left sitting on a hot stove for too long.

"[26] Rachel Cooke of the New Statesman was less complimentary in her review, suggesting that Mercurio struggled to give the audience a logical resolution – leading her to call series five “Jed Mercurio’s ropiest bit of work ever sent our way.”[27] Brian Donaldson of The List had solid praise for the actors, but felt the series's plot was somewhat vague; he was critical of the script, stating: “The writing leaves many of them discussing the gravest of matters as though they're reading the contents on the back of a shampoo sachet.”[28]