Episode 10

Flooding London is a simple threat to understand, which is what makes it so good. A group of environmental terrorists takes the Thames Barrier and holds it against the storm surge heading for the capital. The government can have it back when it publishes a secret document called Aftermath. That's the whole pitch, and it's plenty.

The problem is that MI5 can't find the document. The Deputy Prime Minister, with the PM away, clearly knows more than she's saying, and a barrier full of armed people isn't something you can just storm. Meanwhile Adam Carter, who's been carrying post-traumatic stress through the series, walks into the facility to negotiate, and the attack hits at the worst moment. He's out of it, and the job falls to Ros Myers.

Ros's way in isn't a gun or a gadget. She has to get inside the barrier and convince the terrorists that MI5 has won, that the document is about to be published, so they'll stand down before the surge arrives. But it's the quietest sort of heroism the show gets to. A woman in a room full of armed true believers, telling them the story they want to hear while the water presses at the gates outside.

David Farr and David Wolstencroft wrote it, Julian Holmes directed. Peter Firth is Harry Pearce, Rupert Penry-Jones is Adam, Hermione Norris is Ros, and Miranda Raison is Jo Portman. The rest of the ranks: Raza Jaffrey as Zafar, James Dicker as Wesley Carter, Derek Hutchinson as Robert Cash, Stewart Wright doubling as Alan Jackson and a Barrier Guide, Anthony Flanagan as Craig Fletcher and Leila Borris as the Female Terrorist.

The episode keeps the two pressures running in parallel, the water rising outside and the lies stacking up inside, and both come to a head at once. The performance that stays with you is Ros's. She walks in with nothing but a story, and she makes the people holding the barrier believe they've already won.