Episode 9

A known mercenary, Robert Morgan, is spotted at a disused Soviet arms dump that should have been sealed and forgotten. The dump is supposed to be forgotten, which is exactly why the sighting matters. MI5 has to bring him in, and the episode asks what the service is allowed to do to get the information out of him. Everyone knows that questioning is where the trouble starts. The options on the table include blackmailing innocent people and threatening a suspect's family. The hour leaves both there, unsanitised, and lets the viewer decide where the line is.

Olga Sosnovska's Fiona Carter and Rupert Penry-Jones's Adam Carter are the ones doing the dirty work, and the episode watches them try to hold on to the difference between method and madness. Adam and Fiona carry the weight of the orders, and the hour watches what that does to them. The episode is at its strongest when it stops explaining and simply shows the cost. David Oyelowo's Danny Hunter, Shauna Macdonald's Sam Buxton, Rory MacGregor's Colin Wells, Hugh Simon's Malcolm Wynn-Jones and Nila Aalia's Juliet Taylor make up the team around them. Malcolm Wynn-Jones, Hugh Simon's quiet fixer, is the kind of presence that makes the Thames House scenes work. Julie Etchingham appears as a Sky newsreader. Peter Firth is Harry, who gives the orders and lets other people carry them. Rupert Walters and David Wolstencroft wrote it; Alrick Riley directed.

Spooks has always been willing to ask how far the state should go. This episode is the question without the comfortable answer. It leaves you a little less sure about the people on the right side, and it is better for it. The questions it raises don't get answered, and the hour is better for leaving them open. And the refusal to wrap things up is the bravest thing about it. It sits with you after the credits, and not comfortably.