Episode 9

Protecting a war crimes witness is the kind of job MI5 gets handed when everyone else has already said no. The witness here is Niko Grecic, once the right-hand man of a Serbian called Tezlia, who is now the one on trial. Keep Grecic alive long enough to testify and the case can proceed. That's the brief Adam Carter, Jo and Zafar Younis are given, and it looks bad from the start, because the man hunting Grecic is Richard Dempsey.

Dempsey used to be MI6. Then he turned on the agencies and became the enemy who knows where the doors are. He wants Grecic dead before he can talk, and he knows exactly how the people protecting him work. The episode runs on that nastiness: the bodyguards and the assassin trained in the same trade, one step apart. Both sides have read the same files, which is what makes the contest so close. And then Dempsey does something cleverer than shooting. He asks Adam to trust him again.

That's the real trap. Adam has to weigh the man who betrayed them once against the chance that this time Dempsey means it, with Grecic's life sitting on the outcome. Personal history and operational need tangle together, and the decision has to be made in minutes. There's no way to test the offer, and nobody is coming to help, not in the time they've got. It's the sort of dilemma the show does well.

David Wolstencroft and Neil Cross wrote it, Julian Holmes directed. Alec Newman plays Dempsey with the menace the part needs, and Marcel Iures is Grecic, the man everyone is fighting over. Around them: Peter Firth as Harry Pearce, Rupert Penry-Jones as Adam, Raza Jaffrey as Zafar, Gavin Abbott as Burke, Michael Maloney as John Russell, Gugu Mbatha-Raw as Jenny, James Dicker as Wesley Carter and William Key as Andrew Faulkes.

Mbatha-Raw was years away from Belle and San Junipero here, doing her time in the ranks. It's a tight, tense hour about trust as a weapon, and the script is careful never to tell you who's right. Dempsey is the danger, but the question the episode leaves you with is whether Adam can afford to believe him.