Jim Prideaux drives onto a private school's grounds towing a caravan and takes a job as substitute French teacher.

George Smiley returns home after an unsatisfying dinner with Martindale, reflecting bitterly on his forced early retirement from the Circus.

Peter Guilliam appears at Smiley's home and escorts him to meet government advisor Oliver Lacon.

Field agent Ricki Tarr recounts his surveillance of a Soviet trade delegate named Boris across Hong Kong bars and hotels.

Tarr finds a hidden diary left by Irina that describes a Soviet mole codenamed Gerald operating inside British intelligence.

Oliver Lacon formally asks Smiley to investigate the mole in British intelligence and Smiley accepts the mission.

Smiley travels to Oxford and visits Connie Sachs, who confirms that Polyakov is a skilled Soviet operative working for Karla.

Guilliam breaks into the Circus archives and swaps the Operation Testify files with dummy documents to steal them undetected.

Guilliam is pulled aside by Alleline's team and questioned aggressively about Ricki Tarr's location and recent movements.

Smiley interviews former duty officer Sam and deduces that Haydon arrived at the Circus with impossible foreknowledge on the night Operation Testify failed.

Smiley meets Prideaux in a cheap motel and learns that Control identified five mole suspects using nursery rhyme codenames.

Smiley confronts Esterhase and exposes the Witchcraft deception, forcing Esterhase to reveal the location of the secret London safehouse.

Tarr holds a Paris Circus operator at gunpoint and transmits a fake urgent message to Alleline to create a distraction.

Smiley waits in the London safehouse and overhears Haydon talking with Polyakov, confirming beyond doubt that Haydon is the mole.

Haydon is killed by an unknown person on the eve of his planned transfer back to the Soviet Union.

George Smiley is forced into early retirement after the catastrophic failure of Operation Testify in Czechoslovakia. A government advisor named Oliver Lacon recruits Smiley to investigate a suspected Soviet mole inside the Circus, the British intelligence service. With help from his former protégé Peter Guilliam and field agent Ricki Tarr, Smiley pieces together the conspiracy surrounding Operation Witchcraft and the mysterious Soviet source called Merlin. His interviews with key witnesses, including former analyst Connie Sachs and the injured Jim Prideaux, expose the elaborate deception running at the heart of British intelligence. Smiley forces the mole to reveal himself by staging a meeting at a secret London safehouse, where senior officer Bill Haydon is caught conferring with Soviet handler Polyakov. Haydon is arrested but dies before he can be transferred to Russia, leaving Smiley to take temporary charge of the damaged Circus.

The central hunt for the mole from forced retirement to capture.

Roach watches Prideaux arrive at school and befriends the new French teacher, both recognizing each other as fellow loners.

Tarr describes Irina falling in love with him and revealing that she and Boris are both Soviet intelligence operatives.

Irina vanishes in Hong Kong and Tarr learns a Soviet plane departed in a rush with a comatose woman aboard.

Smiley recruits Guilliam for his investigation and sends him to secretly photograph restricted documents inside the Circus.

Guilliam discovers that the Circus duty log records from the night Tarr sent his message about Irina have been removed.

Smiley establishes his investigation headquarters at the Hotel Islay under the false name Barraclough.

Smiley reads through the night studying Operation Witchcraft files and the intelligence reports attributed to the source called Merlin.

Smiley interviews Max, who reveals Prideaux changed the mission plan and sent Max away just before the Brno ambush.

Smiley speaks with journalist Jerry Westerby, who reports that Soviet forces were already positioned in the Brno forest before Operation Testify.

Smiley reads Haydon's glowing 1937 letter recommending Prideaux to join the Circus, which hints at a deep emotional bond between them.

Prideaux tells Smiley that after being captured and tortured in Czechoslovakia, he eventually revealed everything he knew to his interrogators.

Alleline and Bland arrive at the London safehouse where Polyakov claims diplomatic immunity to avoid arrest.

Alleline takes indefinite leave from the Circus and Smiley is asked to assume temporary leadership of the organization.

Haydon confesses to Smiley that he grew disillusioned with the capitalist West and chose to work for the Soviet East.

Prideaux returns to his teaching job at the school while Roach convinces himself the gun incident was just a dream.

At a private school, the lonely schoolboy Roach befriends the equally isolated new teacher Jim Prideaux, unaware of his former life as a spy. Meanwhile, Ricki Tarr's doomed romance with Soviet courier Irina in Hong Kong sets the mole investigation in motion when she is snatched and her diary reveals the existence of a traitor called Gerald. Smiley methodically builds his case by recruiting Guilliam, establishing a secret base of operations, and studying the Operation Witchcraft files across long nights of reading. His interviews with witnesses such as Max and journalist Jerry Westerby reveal that the Brno mission was a deliberate trap, while a letter from Haydon's past hints at the depth of his bond with Prideaux. After the arrest, Smiley is left to manage the institutional fallout while Haydon confesses his disillusionment with the West, and Prideaux quietly returns to the school where Roach chooses to forget what he saw.

The secondary threads of investigation, betrayal, and aftermath.

Tarr sends a coded message to the Circus warning that Irina holds information crucial to the well-being of British intelligence.

Mendel informs Smiley that Prideaux has been tracked down and is now working as a schoolteacher at a private school.

Smiley first encounters the Operation Witchcraft documents and presses Lacon for access to more classified files on the operation.

Smiley discovers secret budget records showing Alleline has quietly funded an undisclosed London house for Operation Witchcraft.

Smiley and Guilliam learn that Irina and Boris were likely executed by the Soviets but decide not to tell Tarr.

Smiley tells Guilliam about his 1955 meeting with Soviet spymaster Karla in India, during which Karla silently stole his engraved cigarette lighter.

Guilliam tells Smiley that rumors of Haydon's affair with Ann have spread throughout the entire Circus, reaching even Tarr.

Roach becomes seriously ill with anxiety after secretly witnessing Prideaux assemble and load a gun in the school chapel.

Smiley warns Minister Sercombe that sharing Operation Witchcraft with American intelligence agencies would hand the Soviets an undetectable deception channel.

Smiley deduces from multiple witness accounts that Operation Testify was a deliberate Soviet trap designed to protect the mole's identity.

Prideaux recalls that his Brno interrogators already knew about Control's rotten-apple theory, confirming a high-level Circus traitor had briefed them in advance.

Smiley plays recordings of Haydon and Polyakov's safehouse conversations to Alleline and Bland, proving the conspiracy beyond denial.

Haydon reveals to Smiley that Karla personally ordered the affair with Ann as a scheme to distract Smiley from suspecting him.

Haydon admits to Smiley that he deliberately sent Prideaux into the Operation Testify trap to protect his own identity as the mole.

Ann arrives by car to collect Smiley from the train station after Haydon's death, signaling a quiet and ambiguous reconciliation.

The mole investigation turns on a series of decisive moments, beginning with Tarr's warning message about Irina and Smiley's discovery of secret budget records buried inside Operation Witchcraft's files. Smiley and Guilliam make morally uncomfortable decisions, including withholding news of Irina's death from Tarr, while Smiley recounts his 1955 encounter with the Soviet spymaster Karla, who stole his engraved lighter in a silent act of dominance. Young Roach stumbles into the dangerous world of his teacher when he witnesses Prideaux loading a gun, while Smiley builds his case through accumulating witness accounts that Operation Testify was a deliberately set trap. The recordings made inside the safehouse blow the conspiracy wide open, and Haydon's final confession to Smiley delivers the two most devastating truths: that his affair with Ann was a scheme personally ordered by Karla, and that he knowingly sent his closest friend Prideaux into the Soviet ambush to protect himself. The novel closes with Ann arriving to collect Smiley from the train station, a quiet and ambiguous reconciliation in the shadow of everything that has been lost.