Episode 3

William Sampson, an MP, crosses the floor to join The British Way, a far-right party on the rise. Crossing the floor is a big enough deal in normal times; in these times it looks like a declaration. For MI5 that is not a news story. It is a threat to democracy itself, the kind that doesn't need explosives to do damage. Adam and Fiona go undercover to get inside the party and destroy its campaign from within.

The infiltration is the heart of the hour. Every conversation is a risk, every alliance a potential leak, and the episode keeps that tension wound tight. And every rally or handshake is a chance to be caught. The party's campaign is the target, and taking it apart from the inside is slow, patient work. Rupert Graves plays Sampson with the smooth menace the role needs. Sampson is the kind of politician who sounds reasonable until you listen properly. David Threlfall's Keith Moran and Mark Flitton's Brian Thorn give the party its muscle. Nicola Walker's Ruth Evershed, Raza Jaffrey's Zafar, Anna Chancellor's Juliet Shaw, James Dicker's Wesley Carter and Joanna Burnett's reporter fill out the world around the operation. Olga Sosnovska's Fiona and Rupert Penry-Jones's Adam are the two out in the cold, and the hour watches them work without a safety net. Peter Firth is Harry, running the show from a safe distance. Ben Richards and David Wolstencroft wrote it; Alrick Riley directed.

The danger here is domestic, which makes it the most frightening kind. There are no foreign capitals here and no bombs. The threat is a party dressing itself up as respectable while the state watches it grow. It's the sort of threat Spooks handles chillingly well, and the ending leaves you with the sense that the fight is only just beginning.