The novel interweaves past and present as a retired spy reckons with Cold War operations he helped carry out decades earlier.
Peter Guillam, a retired British intelligence officer, lives on a remote farmstead in Brittany, France, with his tenant Catherine and her young daughter Isabelle. He receives a summons from his former employer, the British Secret Intelligence Service, known internally as the Circus, ordering him to London to address a historical case. Born to an Anglo-French father who died in World War II and a Breton farmer's daughter, Peter was recruited for courier missions into the Soviet Bloc before the spymaster George Smiley invited him to join the Circus as his protégé.
At the Service's new London headquarters, Peter meets Bunny, the chief lawyer, and Laura, who handles historical cases. Bunny asks about Operation Windfall, a deception operation mounted against the East German Stasi (the secret police of Communist East Germany) in the late 1950s and early 1960s. Peter feigns amnesia. Bunny explains that Christoph Leamas, the son of Peter's late colleague Alec Leamas, and Karen Gold, the daughter of Alec's girlfriend Elizabeth Gold, are suing the Service for full disclosure, punitive damages, and a public apology. Both Alec and Elizabeth were shot dead at the Berlin Wall, and their children blame the Circus. The Windfall files have vanished, with destruction certificates bearing signatures that resemble Peter's handwriting. Bunny confiscates Peter's British passport.
Laura takes Peter to the Stables, a Bloomsbury safe house that served as Windfall's operational base. The elderly safe house keeper Millie McCraig is still in residence, the premises preserved like a time capsule. Laura discovers files Peter hid inside hollowed-out books and presses him about a sub-source called Tulip, whose real name she already knows: Doris Gamp.
Installed as a file reviewer and lodged in a surveilled flat at Dolphin Square, Peter revisits Windfall's origins. In the late 1950s, a trainee at the Circus's Berlin Station discovered a Minox film cartridge containing high-grade intelligence on Soviet and Stasi operations. Alec Leamas, who ran Berlin Station, suspected a traitor inside the Circus and reported the find only to Smiley. He traced the source to Dr. Karl Riemeck, a physician to the East German elite, codenamed Mayflower. Riemeck revealed that the true origin of the intelligence was his patient Doris Gamp, codenamed Tulip, a Stasi employee coerced into serving as mistress to her boss, a senior intelligence director. Disgusted by the regime, Tulip had begun photographing classified documents. Peter was assigned as her blind courier, collecting her film through wordless brush-pass encounters across the Eastern Bloc. During these exchanges, he felt a growing, unspoken connection to her.
When Tulip's husband discovered the missing camera film and confronted her violently, Leamas defied orders and drove into East Germany to extract her. Tulip arrived at the rendezvous with her young son Gustav, refusing to leave without him. Leamas arranged for the boy to be escorted back to Berlin, promising Tulip he would secure Gustav's release, a promise he knew was almost certainly empty. After an arduous crossing into Czechoslovakia on foot, the pair reached Prague, where Peter, posing as Tulip's husband, accompanied her on a flight to Paris. At the Hotel Balkan in Prague, despite the official report describing an uneventful night, Peter and Tulip made love. At Le Bourget airport, Peter blurted "I love you" as she was transferred to a Circus vehicle. She was flown to England; he was sent to Brittany, tormented and suspicious that the exfiltration had gone too smoothly.
Summoned to Camp 4, a Circus safe house in the New Forest, Peter learned that Tulip was dead, found hanging from a tree. The truth was worse: Hans-Dieter Mundt, a Stasi operations chief, had lured her out with a note promising to reunite her with Gustav, then strangled her and staged the hanging. The order had come from Moscow Centre, the Soviet intelligence headquarters, relayed through the Stasi to protect a mole inside the Circus. Smiley revealed the deal he had struck with the captured Mundt: In exchange for suppressing evidence of his crime, Mundt would spy for the Circus, his rising career giving him access to eventually identify the traitor. Peter was sworn to secrecy; officially, Tulip had taken her own life.
From this bargain flowed the Windfall operation. Mundt, now codenamed Windfall, provided excellent intelligence but could not prevent his rival Josef Fiedler from destroying the Circus's agent networks. After the Berlin Wall went up in 1961, the entire Mayflower network was wiped out, and Riemeck was shot trying to escape. Control, the head of the Circus, devised a plan to eliminate Fiedler and protect Mundt, using Leamas, who did not know Mundt was their agent, as the unwitting instrument.
Leamas began a controlled descent into disgrace, making himself an irresistible recruitment target for the opposition. Separately, Smiley asked Peter to befriend Elizabeth Gold, a young Communist woman known as Liz. Using a French cover identity, Peter met Liz through morning runs and steered her toward a job at the same library where Leamas had been placed. The two fell in love, with Peter watching helplessly, bound by secrets he could not share.
The operation reached its catastrophic conclusion at a Stasi tribunal where Fiedler accused Mundt of being a British agent. Fiedler's case was succeeding until Liz was called to testify. Her innocent admission that Smiley had visited her after Alec's disappearance, reassuring her that Alec was doing a wonderful job, destroyed Fiedler's case and exonerated Mundt. Fiedler was condemned. Liz was shot dead at the Berlin Wall; Alec was shot trying to reach her.
In the present, Peter's defense lawyer Tabitha deduces the full truth: Windfall was not a failure but a successful operation with devastating human costs. Documentary proof could collapse the parliamentary inquiry; the proof exists, as Millie has preserved all original Windfall documents in microdots, but she refuses to release them without Smiley's authorization.
Meanwhile, Christoph tracks Peter down. Years earlier, Peter had signed his real name and Brittany address in a condolence book at Alec's Berlin grave, giving Christoph the means to find him. Christoph first confronts Peter near Tower Bridge, demanding one million euros to drop the lawsuit. Days later, he appears at Dolphin Square with a pistol, announcing he has come to kill Peter. Peter challenges him to shoot. Christoph breaks down sobbing over the nearby Embankment parapet, and Peter disarms him, throwing the gun into the Thames.
Peter escapes London using a French passport he had hidden in advance and makes his way to Freiburg, Germany, where his old colleague Jim Prideaux, now teaching at a boarding school, provides Smiley's address. Peter finds Smiley in a university library reading room. Over dinner, Peter recounts everything. Smiley erupts in fury at the Service for scapegoating Peter and vows to instruct Millie to release all documents, seek out Karen Gold, and offer himself as a sworn witness. Reflecting on his motivations, Smiley insists he acted not for patriotism or ideology but for Europe, to lead it out of darkness. He acknowledges the futility visible in hindsight but maintains they were "never pitiless," possessing instead "the larger pity" (362).
In the epilogue, Peter writes from his farmstead. Isabelle, who has long had difficulty with eye contact and speech, has smiled directly into Peter's face, and the family hopes she may one day speak. The first snow of winter has fallen. Peter watches the postman weave up the hill and wonders whether he will have a letter from England.