With the shock ‘Fall the Midwife’ conclusion to Episode 1 still reverberating round Twitter with tectonic effect, and viewers everywhere wearing the expression of someone who looks at the raw piece of meat on their kitchen counter and realises they accidentally put their cat in the oven instead of the roast, you might have thought that Episode 2 couldn’t surprise you with anything so jaw-dropping that it would top Episode 1. Well, that would be a massive underestimation.

And underestimation is the theme this week, for both us at home and the folks at AC-12, who think they’ve got DI Denton (Keeley Hawes) on the ropes. ‘Denton doesn’t know the half of what we’ve got on her,’ Adrian Dunbar (we just have the word ‘BADASS’ scribbled in our notes next to his name) growls, and it looks like he’s right.

Between them, Jed Mercurio and Keeley Hawes have created a character so deeply embedded in the moral grey area that we’ve almost forgotten what black and white are and where the line between them lies.

Little do we realise we’re watching parts locking together for an exquisite Mousetrap-like set up in a final act that is, without question, one of the finest 15 minutes of drama in recent memory. The Beeb should play it every time we have to fork over the licence fee, just to remind us where our money goes. One man Spanish Inquisition Hastings (Dunbar) relentlessly crushes every defence Denton puts up until finally she’s cornered, under arrest. The evidence is overwhelming. Isn’t it?

And then, just like last week, there’s a dramatic drop… but this time of knowledge! On AC-12! BOOM! In between working on a missing persons case that has more to do with the witness transfer than may first seem, sparring with her neighbour, and punching Fleming in the gut, Denton’s been learning about her accusers. And holy shit, she has done her homework.

Hastings’ perilous financial situation is uncovered; Arnott’s less-than-professional visit to Nurse Tindall is revealed; Fleming’s false friendship with Denton is used against her. In one fell swoop of paper, Denton shows that her accusers are no more virtuous than her. The moral line has been worn away by being straddled so vigorously so often.

And just when we think we’ve got a grasp and made up our minds about Denton, the identity of the witness she was transporting is revealed (to AC-12, not to us, dammit), and everything we know, or think we know, collapses. We’ve completely underestimated just how ambitious Line of Duty is. Who is that witness? Why did Arnott tear down Trotman’s picture? Is Denton innocent?

Good grief, this is getting too much. Where’s that chipmunk?

Aired at 9pm on Wednesday 19 February 2014 on BBC Two.

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