After being told that his treatment for cancer has been a success, Sir Anthony Blunt goes to inspect some paintings. One of them, a Triple Portrait previously misattributed to Titian, shows Titian himself and a Venetian Senator. It soon becomes clear that the portrait contains a third figure which has been painted out. Blunt is called away by a phone call from Arthur Chubb, the new MI5 man assigned by his superior Donleavy to debrief him. While the third man is uncovered in the painting, Blunt visits Chubb and is asked to identify the subjects of various photographs. The photos relate to Blunt's past as a KGB spy.
Chubb has begun to take an interest in art, so Blunt takes him to the National Gallery where he provides insight into the meaning of the symbols in Titian's An Allegory of Prudence. Blunt asks Chubb how many people are aware of the fact that he used to be a spy. Chubb tells him that a small number of people do know, and warns him that unless he starts to provide more solid information about other spies, he may be exposed. He was only offered immunity from prosecution, not anonymity.
Blunt arrives at Buckingham Palace to retrieve the Triple Portrait to analyse it further. He is surprised by the arrival of Her Majesty the Queen, whose trip to open a swimming pool has been unexpectedly postponed. She is initially unhappy that he wishes to take the painting away, but eventually acquiesces. She asks if she has many fakes in her collection, but Blunt suggests that they are not fakes as such, merely cases of mistaken attribution. She remarks that in cases where the pedigree of a work is of unimpeachable origin it is highly unlikely that it would ever be openly declared a fake.
Donleavy is actually a secret friend of Blunt and warns him that he will have to be exposed as too many people are becoming suspicious. Chubb goes to see Blunt in the hope of finding out who the 'fifth man' in the Cambridge spy ring was. Blunt counters by telling him that X-rays have revealed that in the Triple Portrait there are traces of a fourth and even a fifth man, but that they are unlikely ever to come to light. Blunt is exposed to the world as a traitor, forced to leave his job and left alone to face the press.
