From Vodzilla.com
The first two seasons of Line of Duty established it as a programme concerned with examining the way in which good cops could get involved in very bad things, for ordinary, ultimately rather banal reasons. In each one, AC12, a police anti-corruption unit, focussed an investigation on a single individual who had fallen foul of their chosen profession and seemingly swapped sides, from hero to villain. Much undercover subterfuge and some intense, nail-biting police interviews led to their misdeeds being uncovered and, by the finales, the cases were, for the most part, resolved.
Regular viewers were expecting Season 3 to repeat the format. This time, Daniel Mays’ character, Sergeant Danny Waldron, was the one spiralling out of control, following the pattern set by Lennie James’ Tony Gates and Keeley Hawes’ magnificent Lindsay Denton, in the previous two seasons. With AC12’s Kate Fleming (Vicky McClure) initially going undercover to investigate Waldron’s firearms unit, viewers settled in for what they assumed would be more of the same winning formula.
But writer Jed Mercurio’s one consistency is his ability to sweep the carpet out from under you, and by the end of the first episode, as Waldron was killed, it was clear we were dealing with something quite different.
A Crime is Afoot 