A British agent infiltrates the enemy by allowing himself to be disgraced at home.

Alec Leamas, the British intelligence officer in charge of espionage in Germany, is sent home after another British agent is killed at the Berlin Wall. The British service, led by Control, wants to eliminate Hans-Dieter Mundt, head of the East German organization. Leamas acts as an alcoholic to undertake the assignment. He obtains a job as an assistant librarian and slowly becomes involved with librarian Nan Perry, a Communist. Sent to jail for fighting, he is released and offered a sum of money to reveal his secrets. Leamas accepts and meets Fiedler, Mundt's assistant. Fiedler, a brilliant Jew who is eager to frame Mundt as a double agent, arranges a secret trubunal with Leamas as the star witness. At the tribunal, Nan Perry is called as a surprise witness, and Leamas realizes that the purpose of the plot was to eliminate Fiedler and strengthen Mundt, who is really a British agent. Mundt arranges for Nan and Leamas to escape over the Berlin Wall, but when Nan, a security risk, has to be shot, Leamas stays behind and is also killed.