At year 1970, the Chairman of the British Intelligence MI6 was forced to resign after he had failed to run the secret mission in Budapest, Hungarian. The failure occurred because it was considered that there was intruder at the high position in British Intelligence.
Before this secret mission, the Chairman of the British Intelligence has already distrusted that there was one of four senior secret agents as Toby Esterhase (David Dencik), Roy Bland (Ciaran Hinds), Percy Alleline (Toby Jones), and also Bill Haydon (Colin Firth) as the spies from Russia.
George smiley (Gary Oldman) as one of the senior British secret agent was forced to retire after the failure mission in Budapest. However, the British government party still believes to George and assigns him to investigate who the intruders actually.