Episode 2

An explosion at a gas processing plant takes Britain's energy supply out for six months, and suddenly the country is shopping for fuel with the lights about to go out. The Russians would happily sell, with strings attached and a price to match. Whitehall turns instead to Tazbekistan, a nation fresh from a coup with a documented taste for torturing its dissidents. MI5's job is to read the negotiators and keep the whole arrangement from blowing up in everyone's faces.

Nobody makes it easy. The CIA, working through Lucas's lover Sarah Caulfield, makes clear that Washington will not tolerate a deal. The Home Secretary passes Harry intelligence about a secret meeting in Switzerland that could ripple far beyond energy prices. And the man at the centre of it, Trade and Industry Secretary Rustam Urazov, wants carte blanche to wipe out his enemies on British soil in return for the gas. Section D is left holding the question of whether that price is worth paying, and what kind of country pays it.

Ben Richards, David Wolstencroft and Zinnie Harris wrote it, with Alrick Riley directing. Nicola Walker's Ruth Evershed, Peter Firth's Harry Pearce and Miranda Raison's Jo Portman do the heavy lifting; Hermione Norris's Ros, Richard Armitage's Lucas, Shazad Latif's Tariq Masood and Robert Glenister's Home Secretary orbit them. Jonny Phillips plays Rustam Urazov, Geoffrey Austin Newland is Jack Hughes, and Lucy le Messurier is credited as Technician 1, which is one of those jobs that sounds made up.

The show has always done its best work when the enemy is a set of choices, and Urazov is the whole dilemma in a suit. The hour stacks the country's needs against its conscience and watches the team bend under the weight. It is politics as siege warfare, and it does not let up. By the end you are less worried about the gas than about the country buying it.