British agent Ricki Tarr, believed to have defected, resurfaces with sensational information: confirmation of a Russian mole in the Service.

The investigation begins, with Peter Guillam burgling the archives for information, while Smiley visits an old comrade whose past knowledge could prove vital to locating the spy.

While Guillam and Tarr come under pressure, Smiley recounts his one meeting with Soviet master spy Karla, who is undoubtedly running the mole inside MI6.

Smiley calls on ex-Circus spy San Collins, who was the duty officer the night Jim Prideaux was shot. Collins confirms the events as Smiley understands them and that Prideaux's good friend Bill Hayden is the senior officer who took charge of the case after all hell broke loose. Smiley subsequently meets with Prideaux to get a firsthand account of the events during the disastrous Operation Testify in Czechoslovakia.

Smiley confronts one of his suspects and gives him the results of his investigations, which could prove a considerable embarrassment to those concerned.

Smiley springs his trap and catches the mole, who confesses how and why it happened, and the answers ultimately rob Smiley of any sense of victory.