Episode 4

Fresh testimony sends AC-12 back on the trail of the Caddy, a corrupt officer with links to organised crime. It is a hunt the unit thought was finished, and the new evidence gives it a second wind. But the deeper the team digs, the more uncomfortable the questions become. The evidence starts to suggest that the Caddy may be inside AC-12 itself, and that is a thought nobody in the room wants to hold. The unit has been here before, and the memories are not pleasant ones.

The episode turns the screw with real patience. The fresh accounts relaunch the hunt, and the unit has to confront the possibility that the enemy is sitting at the next desk. That is the show's favourite kind of horror, and it plays it well. Denton's return to the story adds another thread, and the familiar faces all start looking slightly different under the new light. Hastings presides over a team that no longer trusts itself, and he knows it. Every file, every interview, every loyalty is now in question, and the show lets that do its work.

Jed Mercurio wrote it, John Strickland directed. Keeley Hawes returns as Denton, and the cast around her is as strong as ever: Martin Compston's Arnott, Vicky McClure's Fleming, Craig Parkinson's Cottan, Polly Walker's Biggaloe, Adrian Dunbar's Hastings, Arsher Ali's Bains, Aiysha Hart's Railston, Jonas Armstrong's Nash and Maya Sondhi's Bindra. The cast has rarely been better deployed, and the tension is the product of that.

The constant looking over the shoulder, the feeling that the threat is across the desk rather than outside the station gates, is what the series does best. This episode is a reminder of how much mileage there is in that fear. But the Caddy is out there somewhere. The only question is which room he works in, and the answer, when it comes, will change the unit's idea of itself.