Alec Leamas, the head of the Berlin station for the British secret service, waits at a checkpoint on the Berlin Wall for his agent Karl Riemeck, to cross over safely to the west. He is shot and killed by East German guards, and is one of Leamas's last agents to be killed by Hans-Dieter Mundt, the head of the East German secret service, the Abteilung.
Leamas is assigned by Control, the head of the British intelligence agency "Circus" , to kill Mundt. To carry out this plan, Leamas takes on a double identity, becoming an alcoholic wreck working at the bank. Later, an employment agent sends him to work at the library, where he continues his alcoholic habits and meets LIz, a Jewish woman and member of the Communist Party. They have an affair, and LIz falls in love with Leamas.
After getting out of jail(for punching someone), he is approached by Eastern German spies in London, Ashe and Kiever, who recruit him as a defector. They offer him money and an additional annual pay for information he gives about his services.
Leamus travels to the Hague, where he is interviewed by a Russian agent named Peters. Leamus reveals information about his initial contact with Riemerk and the intelligence he provided. He talks about other operational activities he did during his past mission.
Leamas meets Fiedler, the second in command of the Abteilung and its best interragator, who asks Leamas about the details of his service and philosophy. Leamas is intensely interragated and questioned. Fiedler later tells Leamas he suspects Mundt of being a British agent.
Fiedler and Leamas are arrested, and interrogated by Mundt. Leamus kills a soldier. Leamas is rescued; in the hospital Fiedler says there will be a Tribunal to see if Mundt is a British spy. Liz and Fiedler testify, and Leamas realizes the truth: Mundt is London's man, and London sent him on the mission to frame and kill Fiedler, who suspected Mundt, and to protect Mundt. Later, Fiedler helps Liz run away with Leamas, but they are both shot before they can make it over the Berlin Wall.