Quiller, an agent working for British Intelligence, is sent to Berlin to meet with Pol, another operative. Pol tells Quiller that Kenneth Lindsay Jones, a fellow agent and friend of Quiller's, was killed two days earlier by a neo-Nazi cell operating out of Berlin. Quiller's assignment is to take over where Jones left off. He is to infiltrate the cell, find their headquarters and expose its members. Quiller accepts the mission, taking the information collected by Jones before his death, but refusing cover, as did Jones. His plan is to let the opposition know he is looking for them, and force them to reveal themselves
Following the clues left by Jones, Quiller visits a bowling alley, then a swimming pool, making sure to mention Jones' name. He then follows the last lead, a newspaper report of an ex-Nazi found working at a children's school. Posing as a journalist, Quiller meets some of the teachers, including Inge. He offers her a ride home, where he tries to find out any information she may have on resurgent Nazi activity in Berlin. After leaving her apartment he has words with a group of men who appear to be following him.
Leaving his hotel, Quiller's leg is scratched by a man carrying a suitcase. After managing to lose the agent assigned by Pol to watch over him, Quiller realises he has been drugged. When he awakens, the leader of the cell, a man named Oktober, interrogates him. Having tried unsuccessfully to escape, Quiller is injected with various drugs to try and make him talk. By concentrating on Inge he is able to avoid giving anything away. Frustrated, Oktober orders that Quiller be sedated again and dumped in the city.
Quiller awakens and, realising he is being followed, steals a taxi and eludes Oktober's men. He checks into a seedy hotel and phones Inge to make an appointment to see her. The next day he is taken for a second meeting with Pol, who explains that he must try and get close enough to Oktober to get his exact whereabouts, but not so close that he can be recaptured and made to reveal details of Pol's operations in Berlin.
Quiller meets Inge and tells her who he really is and about his operation. They go to bed together. Later she tells him that she knows of someone who may be able to help him, a friend of her father's who was once involved with the neo-Nazi group. The man turns out to be the swimming pool manager he had met before. That night, together with a colleague from Inge's school, Quiller is taken to Oktober's base, an abandoned castle. Quiller decides to investigate, giving Inge a phone number to call if he doesn't return in twenty minutes.
Quiller finds Oktober's men waiting for him in the castle grounds. They have captured Inge and give him until daybreak to decide whether to betray his side and save her, or be true to his mission and let her be killed. Quiller walks through Berlin, at various stages trying to escape, but Oktober's men make it impossible for him to contact Pol. He eventually returns to his hotel, to find that his room has been searched and the phone disabled. He goes to his car and finds that a bomb has been placed under it. He removes it, places it on the hood of the car, and then turns the motor on. The vibrations make the explosive slowly move forwards. Quiller gets out of the garage before the bomb falls off the car and explodes. Oktober's men, now believing him dead, leave the hotel.
Quiller gives Pol the details of Oktober's base and eventually all the men are captured, though Inge is nowhere to be found. He finds her back at the school, where she claims that Oktober let her go. She says that she tried using the phone number he gave her, but that it didn't work. He asks her if she ever met a man named Jones. She says no. Quiller walks away.
