At a student party at Cambridge University in 1934, 'Kim' Philby comes to the aid of Jewish student Miriam when she is insulted by an anti-semitic student, Givens. Another student, Guy Burgess, ridicules Givens and defuses the situation. Philby and Miriam spend the night together. Philby and Burgess have breakfast together the next morning in the dining hall. When they discover that the catering staff will be let off during the Summer holiday without pay, Burgess promises to do something about it, much to Givens' disgust.

Burgess and University Fellow Anthony Blunt are working as recruiters for the KGB and think that Philby and his friend Donald Maclean are suitable candidates to help them fight for the common man and against Fascism. Blunt invites Philby to High Table for dinner. When the waiters stage a strike, Philby makes a point of getting his own food and is applauded by many of the students. Givens walks out with some of his right-wing friends. He gives Michael, a waiter, some money and later Michael and Burgess are discovered by the college porter having sex. Michael is dismissed while Guy is told that as he is part of the establishment he won't suffer any retribution. Philby runs in to Michael who admits that he was paid a year's salary by Givens to set Burgess up.

Otto, Blunt's KGB controller, arranges a test to see if Philby and Maclean are prepared to join their cause. They both pass, but Maclean remains non-committal until the death of his father. Miriam starts to suspect that Philby and Maclean are up to something.

At the end of term, Burgess, Philby and Maclean jump into the river Cam to celebrate their new lives working for Moscow. Philby goes to Vienna to deliver a letter for Otto. He stays with Litzi Friedman and quickly falls in love with her. He tries to help her smuggle a Jewish Communist out of the country, but the old man is captured and shot. Philby marries Litzi, who is then able to leave the country as a British citizen. Back in London, Philby is convinced by Blunt and Otto that he must divorce Litzi as she is a well-known Communist; with her by his side, Philby and his friends could never accomplish their plan to infiltrate the British establishment.