In 1945, Konstantin Volkov, a Colonel in the Russian Consulate in Istanbul, offers to reveal the names of two men in the employ of the KGB working in the Foreign Office and of one KGB man in the British Secret Service, in exchange for money and political asylum. The man sent to debrief him is Harold 'Kim' Philby, who, unknown to the British Secret Service, is in fact the KGB spy. He informs Moscow and Volkov disappears. When Philby is given a top post in Washington, he is told that a partially decrypted message sent to Moscow from America suggests that there is a security leak in the British Embassy in Washington. One of the men on the list of suspects is Donald Maclean, who has since been posted to Cairo.

Following a series of embarrassing incidents involving his homosexual affairs, lax security and serious bouts of drinking, Guy Burgess is removed from his London post in the Foreign Office and sent to Washington. Philby invites his friend to stay with him and his family, despite his wife's objections. In Cairo, Donald Maclean suffers a nervous breakdown when he is labelled a security risk as a result of his excessive drinking and is sent home to recover. When he recovers, he is given the American desk in London to run. Security over Soviet leaks is raised after the Russians explode their first atomic bomb.

Maclean is unnerved when he comes under renewed suspicion after computer analysis is able to link his activities in America with the Washington leaks. Philby sends Burgess to arrange his defection since he is sure that Maclean would crack under the strain of interrogation. Philby learns that Maclean is to be picked up after the weekend, so, on the Friday, Burgess and Maclean leave the country by ship.

Philby is suspected of having tipped off his two old university friends and is subjected to a long vetting process. Unable to find any concrete evidence and under pressure to provide quick answers to the claims of further treachery, his superiors officially clear him of any charges. At a press conference, Philby denies being the 'third man'. Philby is relieved of most of his high level duties and remains under observation. Eight years later he eventually defected as well.