In 1970s England, the head of MI6, Control (Hurt), dispatches an agent to meet with a Hungarian general who knows the identity of a Soviet spy in the organisation's ranks. However, the mission goes wrong and the general dies before he can reveal the information. Undersecretary Oliver Lacon (McBurney) calls veteran agent George Smiley (Oldman, in his first Oscar-nominated performance) back from forced retirement to ferret out the mole and stop the flow of vital British secrets to the Russians.