Lust

Lorraine Johnstone has a dangerous obsession with a known pimp, and the episode's engine is the moment the squad finds out. Jason Grant is the man, played by Philip Glenister with the easy menace of someone who has never had to worry about the consequences. But an affair between a vice officer and a pimp is the kind of scandal that takes careers with it, and everyone in the unit knows it. The unit runs on discretion, and discretion has a shelf life.

Both Vickers and Joe Robinson stand to lose if the affair becomes public, which should make them allies. It makes them enemies instead. Vickers moves first, trying to use Lorraine for his own ends and sacrifice her when it suits him, and the calculation fails in a way he never saw coming. It exposes what Joe has long suspected about him: the dark heart of his corruption. He had been waiting for the proof, and Vickers hands it to him.

The episode is a tightly wound duel between the two senior men, with Lorraine as the piece neither of them should be playing. Tim Pigott-Smith and David Harewood have been circling each other all series, and here they finally get the argument the show has been building towards. Tamzin Malleson's Johnstone is the tragedy in the middle, and the episode treats her as a person rather than a plot device, which is the decent thing to do and not always the obvious one.

Hutchins, Morgan, Parkes and Cole hold the unit together while their seniors tear at each other, and Connie Walker's Bryonie and David Alexander's Alec fill out the pimp's world.

Clive Bradley wrote it, Gavin Millar directed. It is the most openly nasty hour of the final series, and the title earns its keep twice over: lust ruins Johnstone, and it ruins the men who thought they could use it.