A year after he was forced into retirement, ex-spy George Smiley (Gary Oldman) is called back by a Cabinet office official when information comes to him that there may be a Soviet spy, a mole, at the top of the British Secret Service. Smiley had been forced out along with Control (Sir John Hurt), the head of the spy agency, after a disastrous mission in Hungary where a colleague, Jim Prideaux (Mark Strong), was shot. It was also an unhealthy time in the Secret Service, known affectionately by its members as the "Circus", with several senior officers having developed a new source of information in the U.S.S.R., but refusing to share that person's identity. Smiley agrees to return, and in the course of his examination, learns that the secret Soviet source has become the mainstay of the service, one that they soon plan to use to get at U.S. Intelligence information. Smiley soon realizes that the Soviets have turned the service inside out.