In Tinker, Tailor, Soldier, Spy, retired intelligence officer George Smiley (Gary Oldman) has been brought in by a government minister to investigate warnings of a mole at the top of the British intelligence service, referred to colloquially as "the Circus." Smiley conducts an exhaustive investigation culminating in a trap that snares Bill Haydon (Colin Firth), a top Circus official. Haydon had been having an affair with Smiley's wife, Ann, and it's hinted that he may have been lovers with Jim Prideaux (Mark Strong), an agent he betrayed. Haydon is arrested, but Prideaux kills him while he's in custody to avenge his betrayal. Smiley is reinstated at the Circus, this time at its head.
In the end, it doesn't matter that the mole proved to be Haydon. It could have been any of the potential suspects at the top of Circus, since what mattered is how Haydon was able to control the investigation, tearing out pages in a logbook and having Prideaux shot and captured before he learned the truth. The moral, in other words, is not "don't trust the person having an affair with your wife." It's "don't trust anyone."
More importantly, it's about what living with that constant high-level of apprehension feels like — and what kind of person it takes to be attuned to all that doubt yet not be paralyzed by it. Smiley is not meant to be an aspirational figure like Bond. He's meant to show what le Carré thought it took to get the job done.
