When a disgraced spy surfaces with information concerning a double agent at the top of the British secret service, George Smiley (Gary Oldman), an ex-MI6 agent, is drawn back into the murky field of espionage. Tasked with investigating which of his trusted former colleagues has chosen to betray him and their country, Smiley narrows his search to four suspects all experienced, skilled and successful agents.

However, past histories, rivalries and friendships make it far from easy to pinpoint the man who is eating away at the heart of the British establishment.