Episode 4

Two men walk out of prison, and London should be scared of both of them, for different reasons. One is an undercover MI5 asset. The other is an impressionable would-be suicide bomber, radicalised inside, who has decided the pair of them are going to attack the city. The team has to stop them, and the timing has to be perfect: too early and the asset's cover goes, too late and people die. There is a version of this story that rushes, and the show does not make that mistake.

The episode is a countdown, and it knows it. There is no mystery to solve and no traitor to unmask. The only question is whether the team can step in at the eleventh hour without losing the asset or letting the bomber through. You sit with the asset the whole way, knowing what he knows. MI5 has to watch and wait while the two men move through the city, and waiting is the part of the job the team is worst at. It is a tight, single-purpose hour, and the show commits to the wait rather than padding it.

Sean Cook and David Wolstencroft wrote it, Julian Holmes directed. Peter Firth's Harry and Nicola Walker's Ruth steer, with Max Brown's Dimitri, Lara Pulver's Erin, Geoffrey Streatfeild's Calum, Tom Weston-Jones's Sasha, Alice Krige's Elena, Jonathan Hyde's Ilya, Simon Russell Beale's William Towers and William Hope's Jim Coaver filling the ranks. The ranks feel fuller than they have all year, which suits an hour about the whole service pulling in one direction.

The undercover asset's predicament is the sharpest thing here: a man who has to stay undercover so long that the people he is hunting stop being strangers. It is the oldest problem in undercover work, and the episode gives it room to breathe. But the episode keeps its eye on the clock, and the ending works because the wait finally pays off.