Episode 4

Peter Salter is MI5's best officer, and the episode's opening question is whether he's turned. With President Bush due in London, Salter looks very much like a man working to stop the peace, and the team has to decide whether it's all part of his mission or whether he's playing the deepest game of all. The hour keeps every option open, and the audience is left to weigh them alongside the team. The question matters because the stakes are personal as well as political, and the episode keeps both in view. It's a lot for one hour, and the show juggles it with more grace than it gets credit for.

Ellie's problems are closer to home. She is dating a spy and starting to wonder if she can live with it, a question the show treats with more tenderness than it gets credit for. Zoe has her own discovery: Tessa Phillips is up to something, and whatever it is, it isn't in the handbook. Meanwhile Tom's mentor is working undercover at an anti-Bush rally, and his methods are getting dangerously unconventional. The rally scenes have a charge of their own, with the protestors' anger very real and the undercover man's methods sitting badly with it.

The episode has several plates spinning and keeps them all in the air, which is more than many series manage with one. Howard Brenton and David Wolstencroft wrote it, with Rob Bailey directing. Peter Firth's Harry, Jenny Agutter's Tessa, Keeley Hawes's Zoe and Esther Hall's Ellie carry the main weight, while the cast list offers its own small pleasures: Robert Finan's Penguin Man and the Cheeky Girls suggest someone in casting had a sense of humour.

It's an episode about trust, which for spies is the whole ballgame. The Bush visit gives it a deadline, and the Salter question gives it a spine. But whatever the answer turns out to be, the hour earns its keep by making you care which one it is.