In 1951 the art historian and former MI5 agent Anthony Blunt is put in charge of running KGB agents in the UK. He decides to recall his lover and fellow spy Guy Burgess from Washington, as their confederate Donald Maclean is under suspicion and Blunt knows that he will not be able to withstand questioning. Burgess promises to follow orders and help Maclean defect. Burgess goes to see his old university friend Goronwy Rees and asks him to help Blunt in case he has to go to Moscow with Maclean.

Blunt is called to MI5 by his friend Guy Liddell to provide expert advice and learns that Maclean is one of four men on the list of suspects. He discovers that Maclean is due to be picked up after the weekend and so hastily makes plans for Burgess to take him to France. Burgess calls Rees to say goodbye, but finds his wife instead. Clearly drunk, he bids her farewell and tells her that he is planning to make a grand parting gesture.

Blunt receives a letter from Burgess which makes it clear that he will be leaving with Maclean. Blunt is furious, then heads over to Burgess's flat to remove compromising materials left behind. As he burns the papers he recalls the twenty years of their friendship and weeps.

Rees confesses to his wife that he briefly worked as a spy for Burgess in the 1930s and that he thinks Burgess may have defected. She is appalled by the admission and insists he make a clean breast of it to the authorities. Blunt calls him and tries to convince him to keep quiet. Blunt tells him that the authorities will not be very sympathetic, but Rees decides to confess what he knows regardless. Rees makes an appointment to see Guy Liddell at MI5, but they are joined by Blunt, who has already spoken to Liddell in an effort to minimise the damage. Rees eventually makes a full confession anyway, but Liddell is clearly not totally convinced.

Blunt would not be publicly unmasked for another twenty-eight years.