Episode 2

Following the tragedy at the hospital, AC-12 plants Kate Fleming undercover in Lindsay Denton's Missing Persons Unit. It is a sensible move on paper: get someone inside and watch Denton for the weak spot. But Denton is not the sort of officer who waits to be watched. She starts gathering compromising evidence against the anti-corruption team itself, and the operation starts to look a lot less one-sided. The missing persons work is the perfect cover, and Denton knows it.

The episode works best when it shows the two women circling each other. Fleming has to do a real job while pretending to be someone else, and Denton has to decide whether the new arrival is a threat or a tool. Around them, the fallout from the hospital keeps pressing in, with Arnott, O'Neill and Mallick dealing with the wreckage. Hastings watches from above, and the sense that AC-12 might be compromised from within gives everything an extra edge. Both sides are spying on each other, which means nobody gets to feel safe. It aired a week after the series opener, in February 2014, and it moves the season from the shock of the ambush into the slower, nastier business of the inquiry.

Jed Mercurio wrote it, Douglas Mackinnon directed. Keeley Hawes and Vicky McClure do the heavy lifting, and the supporting cast holds the frame: Martin Compston's Arnott, Michael Nardone's O'Neill, Steve Toussaint's Mallick, Adrian Dunbar's Hastings, Tony Pitts's Hargreaves, Christina Chong's Rogerson, and Briege Roche and Gavin Neilly as Georgia's parents, who remind you what the missing persons work is actually about.

An undercover story only works if you believe the danger is real, and this one makes you believe it. The trap being built around Denton may close on someone else entirely. But the clever twist here is that the trap cuts both ways, and the episode walks that line well. It is a fine line, and nobody on screen gets to relax.