In the early 1970s during the Cold War, the head of British Intelligence, Control, resigns after an operation in Budapest, Hungary goes badly wrong. It transpires that Control believed one of four senior figures in the service was in fact a Russian agent - a mole - and the Hungary operation was an attempt to identify which of them it was. Smiley had been forced into retirement by the departure of Control, but is asked by a senior government figure to investigate a story told to him by a rogue...