1973. The Cold War continues to poison international relations. The British secret service is, like those operating in other countries, on maximum alert. After a failed mission in Hungary, the head of M16 is put out to pasture, along with his loyal lieutenant George Smiley.

However, Smiley is soon secretly re-employed on the government's command. There are fears that M16 has been infiltrated by a Soviet double agent. Backed up by Peter Guillam, a young agent, Smiley tries to uncover the mole, but he is soon entangled in old ties linking him to Karla, a formidable Russian spy.

While the mole's identity remains an enigma, Ricki Tarr, an undercover agent working in Turkey, falls in love with Irina, a married woman who claims she has crucial information in her possession. Meanwhile, Smiley learns that his former chief had reduced the suspect list to five names: the ambitious Percy Alleline and Bill Haydon, the charmer Roy Bland, who had always demonstrated loyalty up until then, the zealous Toby Esterhase... and Smiley himself.

In an atmosphere of suspicion, manipulation and manhunts, everyone finds themselves playing a dangerous game that could cost them their lives and throw the world into chaos.

The responses lie beyond each person's limits.