Episode 2

Harry Pearce is convinced that Arkady Katchimov, the head of Russia's intelligence service in Britain, is behind the al-Qaeda bombing that killed Adam Carter. The show has taken its main man off the board, and Harry is not handling it well. He launches an unauthorised investigation, hunting for a non-diplomatic source who can give him something on Katchimov's activities.

Meanwhile the team intercepts an electronic burst that suggests an unidentified Russian submarine in British waters. The Russians, they conclude, may be planning to tap undersea communications cables and launch a denial of service attack that would disable the Internet across Britain, with all the financial and social chaos that implies. It is a big, modern kind of threat, an attack on the plumbing of everyday life rather than on buildings, and the episode handles it with the team working blind, unable to confirm even where the submarine is.

Lucas North is desperate to get back to work. Harry insists he complete the full re-integration process first, and the caution proves justified when it becomes clear that Lucas may be working for the Russians after all. The trust Harry is pouring into him is the most dangerous asset in the episode, and only the audience gets to see both sides of it.

Neil Cross and Ben Richards wrote it, Colm McCarthy directed. Peter Firth plays Harry Pearce, Richard Armitage plays Lucas North, Hermione Norris plays Ros Myers and Miranda Raison plays Jo Portman. Gemma Jones's Connie James, Hugh Simon's Malcolm Wynn-Jones, Alex Lanipekun's Ben Kaplan, Marcia Ashton's Mrs Moore, Paloma Baeza's Elizaveta Starkova and Robert East's Richard Dolby complete the cast.

The submarine is the plot and the mole is the point. Harry is chasing a man he can name while the man he trusts is the one doing the damage. Everyone in the episode is looking in the wrong direction except the audience, which is exactly where Spooks wants you. And the hour leaves you waiting for the other shoe to drop.