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When two British Intelligence agents are murdered in Berlin, Quiller, an American agent, is given the task of finding the leader of a neo-Nazi movement attempting to infect German thinking.  Under the watchful eye of Pol, head of Berlin Control, Quiller follows a newspaper lead and visits a school where a teacher convicted of Nazi war crimes recently hanged himself.

There Quiller is attracted to the teacher's replacement, Inge, and visits her at her apartment.  Upon leaving, he is picked up by the Nazis, drugged, and brought for questioning to their headquarters.  When he refuses, despite torture, to reveal the location of Berlin Control, the Nazi leader, Oktober, orders him dumped into the icy waters of a canal.

Realizing Oktober expects him to report to Berlin Control, Quiller goes instead to Inge and asks for her help.  Her "contact"--her headmistress--leads them both back to Oktober.  The Nazi informs Quiller that he is to be set free, but unless he reveals the location of Berlin Control by daylight, both he and Inge will be murdered.  By deliberately setting off a bomb which has been placed in his rented car, Quiller slips from the scene as the blazing explosion convinces the Nazis that he has been killed.

Once he has reported the address of Oktober's headquarters to Pol, and the Nazis have been quickly rounded up, Quiller learns that Inge is not among the prisoners.  He finds her back at her school; she tells him only that Oktober let her go.  As Quiller leaves, Inge returns to "teaching" the younger generation of Berlin children.