A hardened CIA operative finds himself at the mercy of a precocious 9-year-old girl, having been sent undercover to surveil her family.

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.

Susan Cooper is an unassuming, deskbound CIA analyst, and the unsung hero behind the Agency's most dangerous missions. But when her partner falls off the grid and another top agent is compromised, she volunteers to go deep undercover to infiltrate the world of a deadly arms dealer, and prevent a global crisis.