During the Cold War, ex-smuggler Harry Palmer is enrolled as a reluctant but talented spy. Harry embarks on a dangerous undercover mission in Berlin.

A bomb explodes under Jean's car and only Harry's quick-thinking saves their lives. Harry spots a suspicious stranger tailing them who is identified as Randall, a former agent.

Harry and Jean arrive in Beirut in hot pursuit of Randall, who is holding Professor Dawson. Meanwhile, Dalby holds a secret rendezvous with Stok, a Russian colonel.

Dalby challenges General Cathcart, suspecting American involvement in Dawson's abduction. Harry and Jean head to the Pacific atoll where the neutron bomb is to be tested.

Harry is handed over to the Chinese military where he is subjected to a bewildering regime of mental torture. Dalby's love for Dr Lavotchkin takes its toll on his judgement.

Harry is tormented by what has been planted in his mind as his mission reaches its bloody climax and the stability of the free world is at stake.