When an elite crime squad's lead detective investigates the disappearance of a victim on the first snow of winter, he fears an elusive serial killer may be active again. With the help of a brilliant recruit, the cop must connect decades-old cold cases to the brutal new one if he hopes to outwit this unthinkable evil before the next snowfall.

In the early 1970s during the Cold War, the head of British Intelligence, Control (Sir John Hurt), 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. George Smiley (Gary Oldman) 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 rogue Agent Ricky Tarr (Tom Hardy), that there was a mole. Smiley considers that the failure of the Hungary operation and the continuing success of Operation Witchcraft (an apparent source of significant Soviet Intelligence) confirms this, and takes up the task of finding him.