While at Cambridge university two students, Philby and Maclean, are approached by Intelligence officers and requested to spy. It does not take Philby too long to realise that it is not the British Intelligence services who want them, but the Russians. They agree to spy for the Russians and, once the school year ends, Kim is sent on assignment to Vienna, where he marries committed communist Lettie Friedman. Once back in England, Philby's runner, Blunt, tells him that the marriage is not in the interests of the mission, and Philby has to end it.
The group go underground as part of the establishment: Blunt is close to the royal family, Maclean joins the Foreign Office, Burgess is in MI5 and Philby is a right-wing journalist. Spain tests Burgess and Philby. Burgess's lover Julian Bell goes to die fighting for the republic. However, Burgess can't blow his cover to support him and is affected by this betrayal. Philby, reporting from Spain, fails to carry out orders from his Soviet control, Otto, to assassinate Franco. Philby flees Spain and survives, but his betrayal costs Otto his life.When Stalin signs a non-agression pact with Nazi Germany, all four feel betrayed as their struggles against fascism have been turned upside down.
Paris is burning. In the midst of the turmoil and degradation, Donald Maclean finds love. Melinda is passionate, strongminded, independent, and American. As the Nazis enter the city, Maclean gallantly proposing to Melinda so that she is allowed to flee with the rest of the Embassy, to England. It's a match made in chaos. Back in London, Blunt is being asked uncomfortable questions at MI5 about his Communist past. He is becoming increasingly ill-at-ease with the business of spying and quietly begins to doubt his commitment. None of the others knows, though Philby is aware that all does not seem well with the man who first inspired him to join the Party.
Posted to Washington during the Cold War, Donald Maclean is using visits to his wife in New York for access to the Atomic Energy Commission, passing information to Soviet contact K. Melinda wants him to stop and CIA man Angleton is suspicious after Soviets explode an atomic bomb. Kim Philby goes to America to head off Angleton. Guy Burgess is also in Washington and is dangerously out of control. Maclean returns to London and does the bare minimum for Moscow. Angleton has traced an atomic leak to possible suspects including "Homer", Maclean's code name. Philby can't protect Maclean any longer, Moscow insists he's recalled along with Burgess. Anthony Blunt orders Burgess to accompany Maclean to Moscow but doesn't explain that neither will be coming back.
