Can Harry trust Lucas?

The Spooks are reeling from the events of last night and Harry vows revenge on the chief of the Russian intelligence services (FSB), Arkady Kachimov. But without backing from the British government Harry must find concrete proof that the Russians are trying to destroy Britain.

Ros co-ordinates the investigation and targets a powerful Russian oil tycoon connected to FSB, Beletsky. She blackmails him to uncover the Russians' next move, but Beletsky can only confirm he's been told to shut down his company's servers at a specific time.

Meanwhile, Lucas is desperate to work back on the Grid, but Harry orders him to stay away - concerned about the relationship Lucas forged with Kachimov during his years of incarceration. It seems Harry is proved right when Lucas secretly meets with his Russian handler - none other than his ex-wife, Elizabeta.

Back at MI5, Harry's fears about the Russians are confirmed when a submarine planning to attack Britain's communications is discovered in British waters. The team must get the submarine's locations and secret codes from the Russians, but Lucas is the only person who can infiltrate FSB. Can Harry trust Lucas?