Episode 4

Kate Fleming wants proof that Tony Gates is corrupt, and she wants it badly enough to take risks. She pushes her relationship with him to the limit, spending time with a man her colleagues would happily see ruined. The problem is that nobody outside the room knows what she is doing or why. DC Nigel Morton starts to wonder whether Fleming is the loyal colleague she pretends to be, and that question begins to poison the unit.

This is where the series starts to earn its reputation for paranoia. The real threat to Fleming is the colleague who files a quiet report about her. Her pursuit of evidence becomes a tightrope act between getting what she needs and losing the trust of the people she works with. Arnott and Buckells work the case alongside her, while Hastings keeps the whole machine running and Morton keeps watching. The pressure cooker atmosphere is helped along by the presence of Gates's family, Chloe and Natalie, who make the man harder to hate. It is the quiet kind of damage, and the show understands it better than most.

Jed Mercurio wrote it, Douglas Mackinnon directed. Vicky McClure carries the episode as Fleming, with Lennie James giving Gates a weary charm that makes the whole situation messier. Martin Compston's Arnott, Adrian Dunbar's Hastings, Craig Parkinson's Cottan, Neil Morrissey's Morton, Nigel Boyle's Buckells and Gregory Piper's Pilkington work around her, and Saffron Davies and Jordyn-Eve Davis Greene appear as Chloe and Natalie Gates.

It is a focused piece of writing about how quickly an investigation can turn on its own people. Fleming is doing the right thing for the wrong reasons, and the episode is happy to let the question sit. But the doubt Morton plants is the real damage. Not everything needs resolving by the closing credits. Sometimes the doubt is the point.