The novel introduces George Smiley, a short, bespectacled British intelligence officer whose unremarkable appearance conceals a sharp mind. His career began in 1928 when his Oxford tutor steered him toward a Secret Service front organization. He served under deep cover during the war, building intelligence networks across Europe. His marriage to Lady Ann Sercomb, a beautiful aristocrat, ended when she left him for a Cuban motor racing driver. Now middle-aged and sidelined, Smiley works under Maston, the Ministers' Adviser on Intelligence, a politically calculating bureaucrat.
On a January night, Maston summons Smiley with urgent news: Samuel Fennan, a senior Foreign Office official whom Smiley routinely interviewed two days earlier about an anonymous allegation of Communist Party membership, has died by suicide. Smiley is baffled; the interview was friendly and informal, and he essentially told Fennan he would be cleared. Yet Fennan's typed suicide letter claims his career is ruined. Peter Guillam, a polished colleague on duty that night, reports that Elsa Fennan, Samuel's wife, found the body after returning from the theatre. Maston instructs Smiley to visit Elsa in the morning.
At the Fennan home in Walliston, Surrey, Smiley meets Elsa, a German Jew who survived years in a concentration camp. She is slight and fierce, accusing Smiley of being an agent of an abstract, dehumanizing state. She insists Fennan was deeply upset after the interview, contradicting Smiley's perception. While upstairs, Smiley answers the phone and discovers it is an 8:30 wake-up call from the telephone exchange. A critical realization follows: Elsa told him she is a severe insomniac. An insomniac would not need a wake-up call, and Elsa was not home the previous evening to request one.
Smiley enlists Inspector Mendel, a sharp officer from Special Branch, a police unit that handles security and intelligence matters, to investigate. Mendel learns the call was requested at 7:55 p.m. by Fennan himself. The contradiction is damning: Fennan arranged a morning call but supposedly wrote a suicide note later that night. When Smiley confronts Elsa, she lies, claiming she sometimes uses the exchange as a memory aid but cannot recall what this particular call was for.
Smiley presents his suspicions of murder to Maston, who dismisses them and refuses to involve the police. At his desk, Smiley finds a handwritten letter from Fennan, posted the day before his death, requesting a lunch meeting. Convinced Fennan did not intend to die, Smiley submits his resignation. At home, he finds a tall, fair stranger inside his house. Pretending to be a delivery man, he retreats and notes nearby license plates. He and Mendel trace one car to Adam Scarr, a garage owner, who reveals that a well-dressed foreigner paid him years ago to register a car under Scarr's name. The man collected it on the first and third Tuesdays of each month. While waiting in Scarr's yard, Smiley is attacked from behind and beaten unconscious.
After weeks of hospital recovery, Smiley learns from Mendel that Scarr's only contact number for the foreigner leads to the East German Steel Mission, a trade delegation that vacated its premises on 3 January, the day Fennan died. Guillam explains that East German intelligence runs agents through couriers; the supervising handler, or controller, adopts a cover name chosen by the agent. Smiley connects key facts: The foreigner Scarr called "Blondie" collected his car on the same Tuesday nights Elsa attended the Weybridge Repertory Theatre. Guillam confirms the anonymous letter and suicide note were typed on the same machine but by different hands. Soon after, Scarr's body is pulled from the Thames, ostensibly drowned while drunk.
Mendel investigates the theatre and uncovers the espionage mechanism. Staff report that Elsa attends every other Tuesday, always carrying a music case and sitting beside a man who carries one as well. They deposit the cases in the cloakroom and swap claim tickets, a dead-drop method for passing materials without direct handoff. On 3 January, the man did not appear; a tall, fair foreigner later collected Elsa's music case.
At a private dinner, Smiley reveals the mastermind: Dieter Frey, a brilliant Jewish German student Smiley knew before the war. Fiercely anti-Nazi, Dieter was recruited by British intelligence and became Smiley's finest wartime agent, providing invaluable intelligence from within the German railway system. After the war, Dieter returned to East Germany and headed the Steel Mission. Guillam confirms that a theatre volunteer identified a photograph of Mundt, a young diplomat from the Mission, as both the man who collected Elsa's music case and the intruder in Smiley's house. Mundt is the operational agent; Dieter is the controller.
Smiley visits Elsa again and, combining sympathy with pressure, extracts a confession. She claims Fennan met Dieter at a Bavarian ski resort and was recruited as a spy, with Elsa acting as reluctant courier. When Dieter saw Smiley with Fennan in the park, she says, he assumed betrayal and ordered Mundt to murder Fennan. Mundt killed Fennan and forced Elsa to type the suicide letter. She provides an emergency contact for Mundt, but the leads go cold. Crucially, Foreign Office records reveal that despite extensive access to secret files, Fennan recently took home only unclassified material, wholly inconsistent with active espionage.
Reviewing the case, Smiley reaches a revelation: The only evidence that Fennan was a spy comes from Elsa. If Elsa herself is the spy, every inconsistency resolves. Fennan's innocent file selections suggest he suspected his wife and avoided secret material. The anonymous letter was his way of contacting security. The 8:30 call was a reminder for his planned meeting with Smiley, where he intended to expose her.
To prove his theory, Smiley exploits Dieter's wartime communication codes. During the war, Dieter used postcards of religious images as emergency meeting signals. Smiley sends a postcard of Westminster Abbey to Elsa from Highgate. She takes the bait, purchasing two theatre tickets for a Thursday show at the Sheridan Theatre in Hammersmith and mailing one in a stamped envelope, following the established protocol.
On Thursday evening, the three men converge on the theatre. Elsa sits rigid in the rear stalls. Just before curtain, Mendel spots not Mundt but Dieter himself, limping through the audience. During the intervals, Dieter and Elsa first talk animatedly, then argue, apparently realizing that neither of them initiated the meeting. As the final act ends, Dieter rises, but Elsa remains still. A woman screams. Dieter has silently killed Elsa during the performance. He slips through an emergency exit into the fog.
Mendel pursues Dieter through foggy streets to the Chelsea Embankment. Near Battersea Bridge, Dieter bursts through Smiley and Mendel, knocking them aside. Mendel catches him, but Dieter strikes Mendel with a pistol butt. Smiley shouts Dieter's name. Dieter recognizes him and murmurs a quiet greeting, then strikes Mendel again. As Dieter cocks his weapon, Smiley charges blindly, driving him against the bridge railing. Dieter clutches at Smiley's collar, but Smiley forces him over. Dieter falls silently into the river. Smiley leans over the railing, weeping, anguished that Dieter never fired, that he remembered their friendship even at the end.
In the aftermath, the government suppresses the affair. Smiley declines Maston's offer of a promotion and sends Guillam a full report revealing his deduction that Elsa, not Samuel, was the spy. At a final dinner, Mendel asks why Elsa did it. Smiley speculates that she dreamed of peace but was driven to espionage by the sight of postwar Germany rebuilt in the image of its Nazi past. Of Dieter, he says he wanted "honour . . . and a socialist world," adding: "They dreamed of peace and freedom. Now they're murderers and spies" (156). That night, Smiley catches the midnight plane to Zurich, where Ann waits, and feels "a glimpse of eternity between two worlds" (156). A fellow passenger mistakes the quiet, suddenly smiling little man for a tired executive looking for fun, and finds him rather disgusting.