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 Soviets detonate an atom bomb and take the world by surprise.