Following a routine security check, the officer Samuel Fennan is found dead in what appears to be a suicide. The intelligence agent George Smiley, who had recently investigated him, is quickly designated as indirectly responsible for the tragedy. Suspecting that the case hides something more complex, Smiley conducts his own investigation.
When a letter sent by the deceased arrives after his death, the suicide hypothesis becomes doubtful. The investigation leads Smiley into a world of deception, espionage, and Cold War-related maneuvers, where the British intelligence services and East German agents could be involved.