It's about 'blue on blue crime' - police corruption - the central question being did DI Lindsay Denton leak information which got a witness killed? Either way, she's on remand for it and is suffering horribly, but the show also deals with blue on blue antagonism with different police investigations obstructing one another, plotting and bickering, allowing one gruff Yorkshire cop to bark 'wind yer neck in, son!'
The brilliant thing about the show is following how the thousand tiny strands of the various investigations slowly come together but, as the case itself becomes clearer, the characters alter and shift. There is no certainty here; the people we rooted for in the first episode are now suspicious, and the sinister ones are now sympathetic.
Two weeks ago we could swear Lindsay Denton was guilty. She clubbed her noisy neighbour with a wine bottle, rammed her head into the carpet and then slammed her assistant in the stomach. Yet, now she's surely innocent and has been framed? I almost wept seeing her bend over her dying mother's hospice bed before being pulled back to jail by sullen guards in puffy black jackets.
As for the team of cops who put her in jail, are they bumblers? Are they vindictive? Or are they valiantly trying to find the truth?
Every week a new perspective is revealed and, because the writing is so clever, every shift makes absolute sense. At no point should the viewer recoil because a character has changed, or a twist seems too far-fetched. This happened with Breaking Bad: there was nothing too outlandish for that incredible show. The foundations had been so carefully laid that anything could be stacked on top and it would hold steady. Likewise with Line of Duty: it's strong enough to take varying perspectives and revelations which shredded what you believed last week.
And in amongst these changeable characters is a patchwork of equally varied accents. The show is right to proclaim itself as a decidedly 'British' drama as every region seems to be represented. There is no Estuary English; instead we have brash Yorkshire and bold Northern Irish and blunt Scottish as well as the standard 'saff London' copper voice. Undoubtedly, this 'original British drama' is determined to live up to its name.
Normally, this would be applauded. Ever since Channel 4 started using unintelligible Geordies to do voiceovers it's been the trend to look beyond London and the BBC have made a conscious effort to pull away from that monstrous city, transferring a lot of their programmes to their new base in Salford and even moving their treasured Question Time to Glasgow.
But their efforts at showing themselves a 'British' Broadcasting Corporation, instead of a London one, have met with suspicion.
One website published an article last month saying the BBC has produced a massive increase in 'British themed' programmes since the SNP came to power. It claimed there were 10 shows with 'British' in the title when Jack McConnell was First Minister but under Alex Salmond there have been 208.
It can't be denied they are plentiful these days: The Great British Bake-Off; The Great British Sewing Bee; Great British Railway Journeys; The Great British Year, all followed by a multitude of others with 'Britain' or 'Brit' in the title - though a lot of these are pinned to the First World War and would have appeared with or without an SNP government.
So, in our referendum year, is this a sly attempt to reinforce Britishness or is it simply a genuine effort by the BBC to live up to its name?
Even if there is a sneaky campaign to 'Brittify' some programmes this should have no relevance in the independence debate. Is the Scottish cringe still so violent that we're to believe we're daft enough to be swayed by the credits on a TV show? That The Great British Bake Off has the power to command our floating voters? Are we all face-painted yokels to be enraged by a flimsy show about pastry?
I say Scots are clever enough to make judgements based on facts. I also say anyone who could have their vote changed by watching some old dears decorate a cake with cute Union flags should be disenfranchised anyway.