Force of Nature

Going undercover means giving up the things that make Chappel Chappel, and this episode takes that bargain further than usual. The assignment is a prison for sex offenders, where young women are being pimped to inmates, and the job is to flush out the ringleaders without anyone inside realising who he is. To sell the story he needs a girlfriend, so Hutchins plays the part, and it puts her directly in the sights of the men running the racket.

The prison is a closed little world, and the show has always done closed worlds well. Ian Puleston-Davies's George Randolph is the sort of inmate who runs things from the inside without raising his voice, and the women being moved through the gates have no power and no names worth keeping. Chappel watches them the way he watches everyone, storing details, saying little.

But the deeper he goes, the more the operation starts to look like a mirror. "Heart of darkness" is a phrase that gets waved at this story in the write-ups, and for once it earns its keep: the case takes Chappel somewhere private, and the episode is honest about him not coming back unchanged. It helps that Ken Stott plays the slide without announcing it.

Around him the usual squad machinery turns. Caroline Catz's Hutchins gets the riskiest scenes as the fake girlfriend, and David Harewood's Robinson and Rosie Marcel's Morgan hold the outside together. Katisha Kenyon's Amanda and Daniel Evans's Aaron fill in the racket's edges, William Armstrong is The Governor, Kelly Hunter the prison psychiatrist, and Andrew Bone an officer called Billy who stays in the background.

Clive Bradley, Rob Pursey and Barry Simner wrote it; Justin Chadwick directed. It is one of the series' most uncomfortable hours, and the discomfort is the point. The racket gets dismantled. The cost of dismantling it is the part that lingers.