Episode 7

The opening of a cultural centre in London sounds like an occasion for ribbons and canapés. For MI5 it is cover for something far more delicate. The Syrian foreign minister, Riyad Barzali, wants contact with the Service, but his own secret service watches him so closely that he cannot risk a single quiet conversation. To get to him, Fiona goes undercover as a PR executive working the room.

Fiona is desperate to lead the mission, and the reason is written in her own history. She has a Syrian past, and the closer she gets to Barzali, the more of it surfaces. The operation was hers to win or lose, and everyone in the Grid knows it. Sam Dastor plays Barzali with the careful stillness of a man who knows exactly what his minders would do to him if they had the evidence. His enemy is not MI5's rivals. It is his own side.

The episode works because it keeps both dangers in view at once: the minister reaching out to a service he should not be reaching out to, and the service that would kill him for it. Farook Sukkarieh and Bashir Shalhoob loom around him, and every handshake at the party feels like a test. The cultural centre, all glass and good manners, becomes the least safe room in London.

Raymond Khoury and David Wolstencroft wrote it, Omar Madha directed. Peter Firth's Harry and Anna Chancellor's Juliet anchor the London end, with Nathalie Armin's Joumana, Philip Bowen's Ambassador Wright, James Dicker's Wesley Carter, Nina Fry's Nurse Jenny and Glyn Grain's Immigration Officer filling out the world.

But the episode belongs to Sosnovska's Fiona, who has to smile through the canapés while carrying a lifetime of personal history in her handbag. Watching her do it is the whole point, and it is a good one. The party never stops being a party, which is the trick.