[1] The series follows the actions of Superintendent Ted Hastings (Adrian Dunbar), DI Kate Fleming (Vicky McClure) and DS Steve Arnott (Martin Compston) as they investigate an Organised Crime Group with links to missing undercover officer DS John Corbett (Stephen Graham).

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.

[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]