George Smiley, the aging master spy of the Cold War and once heir apparent to Control, is brought back out of retirement to flush out a top level mole within the Circus. Smiley must travel back through his life and murky workings of the Circus to unravel the net spun by his nemesis Karla 'The Sandman' of the KGB and reveal the identity of the mole before he disappears.
George Smiley's quiet retirement is interrupted when messengers arrive from London Station and call upon George Smiley to come back into the game. Smiley is brought to Sir Oliver and it is revealed that a highly influential mole has been operating out of the Circus for quite some time.
Ricki Tarr, an active agent, recounts a tale to the spy masters that sways Smiley in favour of returning to the Circus to secretly ferret out the mole. Tarr's tale is of The Sandmann, the KGB counterpart and nemesis of Smiley, and of his network in the Circus.
Now that Smiley has rejoined the game he must secretly procure the information and files needed to conduct his investigation. While Peter breaks into the Circus' archives and finds files that bring a whole new perspective to consider, Smiley goes to interview an old friend.
Ricki Tarr becomes unstable and George Smiley must examine his past to find answers. He must examine his personal life as well as his past with the Circus, especially in regard to Control. He also reflects on his meeting with Karla, 'The Sandman'.
Smiley, now fortified with new information, goes to confront Jim Prideaux, an old spy who long since came in from the cold and is now a school teacher. Control, in his last days, seemed to have confided in him and sent him on a mission behind the Iron Curtain to find out who exactly the mole in the Circus was, Tinker, Tailor, Soldier, Rich man, or Poor man.
Smiley now has the information he needs and starts to stir things up with the men of which Control suspected one to be the mole. He is able to eliminate two, himself and one other. The waiting begins, when suddenly the message arrives that Prideaux has disappeared.
The waiting has paid off and the traps closes on Karla's mole. It is agreed to trade him against several English spies who were captured by the KGB, but Prideaux is still not found and he has an open account to settle with the mole. With the command structure of the Circus hollowed out and containment in progress, Smiley cannot come in from the cold but remains to pick up the pieces.