Set in 1974, 1980 and 1983 -- the third novel in the quartet has been left out except for some scenes that furnish flashbacks in the final film -- the stories are based on real events in the region: the first, on the "Moors murders" of five children over 1963-65; the second, on the "Yorkshire Ripper" murders of thirteen women in 1975-80 by one Peter Sutcliffe; and the third on case of Stefan Kiszko, who served sixteen years for a 1975 murder he didn't commit.
"Red Riding Trilogy" uses such elements as scaffolds, but the films are much more concerned with how character unfolds among those trying to untangle events than your usual procedural. Perhaps for this reason, though Grisoni reportedly did make charts to keep the storylines straight as part of his writing process, we probably find little need for that as sets of characters return or fade, come into close-up focus or recede for a time. All this occurs in the context of police corruption and greed, the region's simmering resentment against outside governance, and larger political events of the time -- the mining strikes, unrest related to IRA activities in the North of Ireland and social conditions that spurred the rise of Margaret Thatcher. Though best seen in order, the films are meant to also stand alone, and the first two could. The third one depends too much, I suspect, on the device of flashback for that to really work.
So for example, the first film -- Julian Jarrold's "Red Riding: The Year of Our Lord 1974" -- begins with a scene -- which all three contain in some variation very early on -- of someone driving, traversing the vast bleak, often rain-soaked or fog-blurred hills of the North, establishing the region's remoteness from the buzzing metropolis of the nation's hub. By the time we zoom in, any marks of global connection -- national TV news or the hulking nuclear stacks that loom above the stubby public housing known as "estates" -- seem as blunted in effect here as the outsiders behind the steering wheel. In Jarrold's film that's Eddie Dunford (Andrew Garfield), a cub reporter who's come back home "from the South" -- his older colleagues quickly dismiss him on that basis as a "young Turk" not likely to last a month.