In October 1973, the head of British Intelligence, Control (John Hurt), assigns agent Jim Prideaux (Mark Strong) to travel to Communist Hungary for a critical meeting with a general who had promised valuable information. Tragedy strikes when Prideaux is shot and captured by Soviet agents, igniting an international incident. As a consequence, Control and his loyal associate, George Smiley (Gary Oldman), are forced into retirement, and Control, already in ill health, soon passes away.
With Control gone, Percy Alleline steps in as the new Chief of the Circus, supported by Bill Haydon (Colin Firth) as his right-hand man, along with Roy Bland (Ciaran Hinds) and Toby Esterhase. Their recent endeavors, part of a project known as "Witchcraft," have successfully yielded high-grade Soviet intelligence, a feat that Control and Smiley had always mistrusted. The materials garnered from Witchcraft are shared with American counterparts, leading to a lucrative exchange of intelligence.
Smiley's retirement is abruptly cut short when Oliver Lacon, the civil servant overseeing intelligence, enlists him to probe claims made by Ricki Tarr (Tom Hardy), an MI6 operative alleging the existence of a long-standing mole within British Intelligence. Control had harbored similar suspicions. Operating outside the Circus, Smiley assembles a small team, including Peter Guillam (Benedict Cumberbatch), and sets out to interview those who departed from the Circus around the time he and Control were forced out.
Among those he speaks with is Connie Sachs ([Kathy Burke]), who had been dismissed by Alleline after asserting that Alexei Polyakov, a Soviet cultural attaché in London, might be a mole-handler. Connie is adamant that there is indeed a mole within their ranks and cites Polyakov as the conduit for intelligence leaking to the Kremlin. Connie's warning was disregarded by Alleline, leading to her termination. Another lead is Jerry Westerby, a duty clerk present the night Prideaux was shot. Westerby recalls calling Smiley's home for guidance, only to be answered by Ann, Smiley's wife. This incident sparks Smiley's realization of an affair between Haydon and Ann.
Upon returning home, Smiley discovers Tarr hiding there. Tarr explains that he had been assigned to Istanbul to track down a Soviet agent named Boris. In his hunt, he learns of Boris’s wife, Irina, who also possesses pertinent information. In a strategic maneuver, Tarr extends his stay and engages in an affair with Irina to gain her trust. However, unbeknownst to him, Irina is aware of his true intentions and proposes to exchange crucial information—the existence of a mole sent by the notorious Soviet spymaster Karla—in return for a new life in the West.
Tarr's attempts to relay Irina’s request back to London are thwarted, leading to dire consequences. Both Boris and the British station chief in Istanbul are found murdered, and Tarr is wrongly accused of defection and murder, forcing him into hiding.
Smiley directs Guillam to retrieve the Circus logbook from the night of the crucial call, uncovering that pages have been mysteriously excised—a discovery that suggests Tarr's assertions hold weight. Realizing that Prideaux, contrary to initial reports, is actually alive, Smiley tracks him down. Prideaux reveals that the Hungary mission was specifically aimed at uncovering the mole’s identity. He shares that Control had assigned codenames: "Tinker" for Alleline, "Tailor" for Haydon, "Soldier" for Bland, "Poorman" for Esterhase, and "Beggarman" for Smiley himself.
During his harrowing interrogation, Prideaux is forced to give in under torture, witnessing Irina’s execution in the process. Meanwhile, Smiley uncovers that Alleline, Haydon, Bland, and Esterhase have been clandestinely meeting Polyakov at a safe house, exchanging low-grade British intel to maintain his cover. Yet, one among them—the mole—betrays their trust, supplying high-caliber information that jeopardizes national security.
Employing strategic threats against Esterhase, Smiley pins down the safe house's location and lays a trap, prompting Tarr to announce he knows the mole’s identity. In a tense confrontation, Smiley captures the mole, revealed to be Haydon, who sinisterly divulges that he had seduced Smiley's wife on Karla’s orders to undermine Smiley's suspicions.
As plans unfold for Haydon’s exchange back to the Soviets, Prideaux, fueled by vengeance for his betrayal, takes action and kills Haydon. The resolution sees Smiley returning to the Circus, assuming his rightful position as Chief once more.
