Episode 3

Somewhere in the pipeline is a thermobaric bomb, a weapon that kills with the air itself, and it is being sold to terrorists who mean to use it in London. A thermobaric weapon does not need to be precise; it needs to be close. Section D's answer is Zaf. He goes deep undercover inside an Al-Qaeda cell, the sort of operation that eats a person's life and then asks for seconds. The plan is to get him close enough to the bomb to stop it before it reaches the city.

Adam, working the cell from the outside, learns that the men they are tracking are not planning to survive. It is a suicide mission, and his attempts to keep them away from their target only make everything worse. Events spiral, and the episode does the spiral well: each decision shrinks the options available to the next one.

The tension sits in the gap between the two men. Zaf inside the cell cannot ask questions, and Adam outside cannot act without giving him away. Rupert Penry-Jones and Raza Jaffrey sell that distance beautifully, two friends running the same race in different rooms. The show has done undercover stories a dozen times, and it still finds the room to make this one hurt.

Ben Richards and David Wolstencroft wrote it, Julian Simpson directed. Peter Firth's Harry, Hermione Norris's Ros Myers, Nicola Walker's Ruth and Miranda Raison's Jo Portman run the operation from the Grid, with Aaron Neil's Ibraham, Ofo Uhiara's Michael Johnson, Adhir Kalyan's Samir and Peter Watts's Environmental Officer out in the field.

And the bomb stays the star of the show, in the way that only a weapon nobody wants to see used can be. The hour ends on a note that is not clean, because clean is not what this kind of story produces. It is the right ending for it.