Episode 4

Three Chinese agents land in London with forty-eight hours on the clock, and the whole of Section D is suddenly trying to work out why. Forty-eight hours is long enough to plan and short enough to panic, and the team knows it. The answer, when it starts to take shape, involves a scientific research corporation, a Chinese scientist named Jiang who is the prize of the whole operation, and Ruth undercover in the middle of it.

But Beth is running her own gamble, activating an asset inside the Chinese Security Services to serve as Section D's mole, and the episode keeps asking whether she has pulled the trigger too soon. Lucas, meanwhile, is faced with a decision that could change his course for good. The three threads run together, and the show trusts you to keep up.

Jonathan Brackley and Sam Vincent wrote it, and Paul Whittington directed. Nicola Walker's Ruth carries the undercover leg, and Sophia Myles's Beth owns the risk. Richard Armitage's Lucas takes the quiet road. Peter Firth's Harry, Shazad Latif's Tariq, Max Brown's Dimitri, Iain Glen's Vaughan Edwards, Laila Rouass's Maya Lahan and Simon Russell Beale's Home Secretary surround them, with David Kar Hing Lee as Chinese Operative 1. It is one of those hours where the team is split across three different stories, and each one shows a different side of the job.

The hour is really about choices, and it has the good sense to let them land without announcing them. Ruth's operation and Beth's gamble get their moments, and Lucas's decision lands last, quietly, the way big choices usually do. And none of the choices are loud, which makes them harder to shrug off. It is the sort of episode that feels smaller than it is, until you add up what it changed.