British Intelligence Officer Nicholas Elliott is suspected of helping traitor Kim Philby defect to Russia, and is interrogated by Mi5 Officer, Lily Thomas. Philby arrives in Russia and rather than receive a hero's welcome, he's interrogated by Sergei, a distrustful KGB Officer.

Lily understands that she will get little to nothing from Elliott in an interrogation setting so sets into motion a psychological play that triggers him to unveil something of the secrets he's keeping through action rather than words. Philby travels to Moscow and reveals to Sergei why he's a communist and lethal double agent.

Nicholas Elliott flushes out another suspected traitor by reminding him of a Russian war crime at the end of World War 2, that he feels partly responsible for due to his blind loyalty to Philby at the time. In showing Lily his pain and guilt, he's also trusting her and starting to form an important new friendship and alliance.

Philby appears to either be working for the CIA or against them as he draws in his wary KGB inquisitors. Elliott follows Sir Anthony Blunt, the traitor he flushed out in Episode 3, to a clandestine meeting with a further possible traitor, Lily's boss, Sir Roger Hollis -- Director General of Mi5.

Elliot works with Lily to provoke and manipulate Sir Roger Hollis into showing his cards. In Moscow, Philby's traumatized by the wasteful KGB killing of two CIA agents whom he lured into a trap. Lonely and alone and with no friends to turn to, he hits the bottle and winds up in hospital.

Philby writes Elliott a letter from his hospital bed, in an appeal to their long friendship, suggesting they meet to bury the hatchet. Elliott and Lily realize they're in danger of being played by Philby, and that Hollis is, in fact, most likely innocent. Elliott travels to Berlin at Christmas to meet Philby, but as this friendship ends a new and better one, with Lily, is just beginning.