Episode 6

Patrick McCann walks into a safe house carrying a grenade and a story. The grenade gets him in the door; the story keeps him there. He is the leader of an Irish splinter group, and he says he has intelligence on Asabiyah, a Middle-Eastern terrorist organisation planning a nuclear attack on the British mainland. The price of the information is steep: thirty-six hours free of surveillance, no questions asked. In return, MI5 gets a warning it can't afford to ignore, and the clock starts running the moment he sits down.

The episode runs on a single dilemma. Trusting the IRA, even when the alternative is radioactive, is a hard sell for any service, and McCann knows it. He plays the situation beautifully, and Lorcan Cranitch gives the part the full weight of a man who has spent his life being underestimated. The scenes in the safe house crackle because both sides know the other is lying, and neither can call it.

Matthew Macfadyen's Tom Quinn and the rest of the team have to work out whether to buy what McCann is selling, while Peter Firth's Harry and Henry Goodman's DG argue the price between them. Howard Brenton and David Wolstencroft wrote the script, Andy Wilson directed. Keeley Hawes, Jenny Agutter and Esther Hall fill out the support, with Graeme Mearns's Jed Kelley adding to the pressure. Linda Marlowe, doubling as Madge and the Housekeeper, gives the hour two very different faces.

It's a talky episode in the best sense: the suspense comes from dialogue and threat rather than set pieces, and the hour knows how long to hold a silence. The truth of McCann's story matters less than what the Service decides to do about it. And thirty-six hours is a long time, the kind the episode makes you feel minute by minute.