To Think the Unthinkable:Line of Duty S4.

"This is beginning to feel like a life's work" - Superintendent Ted Hastings

"There are some people there is no immunity from" - Jimmy Lakewell

When a suspected serial killer is caught by DCI Roz Huntley (Thandiwe Newton) and her team, he proves uncooperative in interviews and holes start to appear in the case against him - so guess who are called in to investigate Huntley?

Yes, AC-12 are back. The gang is back in town. Superintendent Ted Hastings (Adrian Dunbar), this time revealing he has some rather old fashioned views with regards to gender equality, DS Steve Arnott (Martin Compston), DS Kate Fleming (Vicky McClure), and, briefly, new team member DS Jamie "not James" Desford (Royce Pierreson), with PC Maneet Bindra (Maya Sondhi) providing vital research back in the office, are thrown, again, into a world of forensics, biometrics, burner phones, and a world in which distrust and suspicion is baked into virtually every exchange.

FC Tim Ifield (Jason Watkins) already has his doubts about Huntley but what are his own motivations? It's clear from the start that Huntley and Ifield, to put it mildly, don't see eye to eye. Huntley is a well respected detective of over two decades service and both her husband, Nick (Lee Ingleby), and her team of DC Jodie Taylor (Claudia Jessie), DS Neil Twyler (Mark Stobbart), and PC Farida Jatri (Anneika Rose) remain, initially, loyal to her.

Those loyalties are severely tested when DCI Ian Buckells (Nigel Boyle) returns to Line of Duty (when Huntley is removed from the case) and when new evidence begins to appear to reveal, as you will have come to expect now with Line of Duty, that the murders Huntley had been investigating, honestly or dishonestly, are linked to crimes and criminals touched on in the previous three series.

Cases are reopened from five years ago. Colleague is turned against colleague, cop against cop, department against department, and wife against husband as AC-12 perilously prise open a world of binbags, balaclavas, thwarted ambition, amputations, dirty protests, dismembered bodies, liaisons in swanky hotel bars, and people being thrown off staircases.

Has ACC Derek Hilton (Paul Higgins) been applying pressure on Huntley to solve the case too quickly or does he, or Huntley, or others, harbour even darker motives? Is Michael Farmer (Scott Reid), an emotionally vulnerable man with clear learning difficulties, being fitted up and, if so, why? What is the involvement of Jimmy Lakewell (Patrick Baladi), a high powered solicitor friend of the Huntley family? And who, mentioned for the first time in Line of Duty, is H?

Series four (the first to air on BBC1 back in March/April 2017 and now on iPlayer) starts, perhaps surprisingly considering the change of channels, a little slower than the first three but once momentum is reached, top speed is relentlessly maintained. We get the big explosive set piece, featuring entirely brand new characters, that comes as standard issue with any series of Line of Duty, we see, as so often, folders full of documents methodically revealed in glass partitioned interview rooms and power tools used for extremely nefarious purposes, and we further investigate the relationship between Hastings, Arnott, and Fleming.

Hastings now like a father, of sorts, to both of them and Arnott and Fleming, not the star crossed lovers it once seemed they might evolve into, but acting like bickering, yet loving, siblings. The directors and writers (Jed Mercurio, of course, and John Strickland) use this as a backdrop to tell a fascinating and labyrinthine story that, ultimately, always, end up with a person in danger of losing everything.

Losing their job, losing their family, losing their reputation, and losing their lives. It is to the credit of all the performers that we find ourselves rooting even for people we know have been dangerously and criminally dishonest. Once characters are pulled into the main body of the story they are always given motivations for their actions, even if those motivations take a long time to become clear.

The main players of series four are ably supported by Gaite Jansen as Hana Reznikova (a waitress and cleaner who is suspected of keeping a dark secret), Harriet Cains as Jade Hopkirk (a rape survivor), Vineeta Rishi as Rupal Pandit, a forensic coordinator, and the return of Arnott's now ex-partner DS Sam Railston (Aiysha Hart) in creating yet another six hours of tense, enjoyable, chilling, and moving drama.

One particularly chilling scene involves us watching a detective ride an office block elevator into what we already know will be extreme danger and one remarkedly moving scene comes when one of the chief protagonists of the story is confronted with the possibility of spending the rest of their life in a wheelchair. Even more moving, extraordinarily so, is a prison visit in which a suspect is urged, for their own good, to tell the truth.

With Line of Duty, it's not always easy to work out who is telling the truth and every time Hastings trots out one of his favourite phrases, "to the letter of the law - THE LETTER", we understand that there are grey areas in every piece of legislature and that these grey areas can be easily exploited by those who wish to do so. Things that look like honest mistakes can have deadly circumstances and, with stakes this high, they tend to do so. The genius of Line of Duty is we are never sure, until the very last moment, who for.