Four-part BBC drama based on the true story of four brilliant young men at the University of Cambridge recruited to spy for the Soviet Union in 1934. Undergraduates Blunt, Burgess, Philby and Maclean bond through their common leaning towards communism, which they see as the only alternative to fascism. They agree to spy for the Soviets and once the university year ends, Philby is sent on assignment to Vienna, where he marries a committed Communist, Litzi 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.