Hauptsturmführer Karl Marsen was a Gestapo officer in the days leading up to World War II. When the Nazis invade Czechoslovakia shortly before the outbreak of World War II, they attempt to capture Dr. Axel Bomasch. The Czech scientist has invented a new type of metal which can withstand immense damage and pressure, and the German high command wants it for use on the U-Boats of the Kriegsmarine. However, having had advance warning of the German invasion, Czech authorities manage to fly Dr. Bomasch to England aboard a plane.
This was despite Bomasch's pleas about his daughter, Anna, who was supposed to meet him at the airfield. German soldiers appeared at her door and arrested her. She was taken to a concentration camp, with the Nazis hoping she knew something about her father's whereabouts. The Gestapo planted Hauptsturmführer Karl Marsen in among the prisoners to befriend Anna and gain her trust.
He claimed to have been a schoolteacher in Germany who was arrested and locked up by the Nazis because of his liberal leanings and their general disdain for intellectuals. He was extremely committed to his role, going to far as to antagonize an SS doctor, which earned him a severe whipping from one of the camp guards. After this, Anna trusted him implicity.
As part of the plan to find Dr. Bomasch, one of the guards was in on Karl's true identity. Karl claimed, to Anna, that the young guard was a former student of his and would help them escape. True enough, the two were soon free of the concentration camp and en route to England. During the course of their trials and tribulations, Karl, despite his fanatical loyalty to the Nazi cause, began to feel some genuine affection for the fugitive scientist's daughter.
Anna did most of the detective work to find her father, learning he was living in the care of Dickie Randall, a British secret agent posing as singer Gus Bennett. Although Anna was joyously reunited with her father, Karl soon revealed his true colors, and led Nazi spies to Randall's home. Randall was knocked unconscious and left for dead while Karl and the agents absconded with Anna and her father to a waiting U-Boat.
By the time they'd returned to Germany, the Wehrmacht had already invaded Poland. War was brewing. They were taken to the headquarters of the Kriegsmarine and ruthlessly interrogated - as much as Karl would allow - by "the controller," Strasser, a high-ranking naval officer. Bomasch refused to make his special metal for the Nazis' submarines. Karl begged Anna to see the world as he did - that Nazism was a force for good and would bring order to a chaotic world, but she considered him brainwashed and pitied rather than hated him.
In the meantime, Dickie Randall's superior in British Intelligence, Dryton, gave him permission to go undercover in Germany to try and rescue Bomasch and his daughter. Using the alias Major Uhlrich Herzog of the Heer, Randall managed to talk his way into Kriegsmarine headquarters, where he made fast friends with Strasser's superor Admiral Hassinger, and, claiming to have been a former lover of Anna's, promised Strasser and Hassinger he could charm her into helping persuade her father to work for the Nazis. For his part, Karl bristled, instantly jealous and suspicious of this mysterious "Herzog."
The plan was to take the Bomasches to Munich via train, with Herzog along for the ride. The suspicious Karl and two Gestapo guards forced their way into the party. During a stopover, Karl called Strasser and learned that no such officer as Uhlrich Herzog existed. He realized "Herzog" must be a British spy. Fortunately, there were two British businessmen abouard the train named Charters and Caldicott, who'd been former schoolmates of Randall's and recognized him and been confused about him being in a German uniform. When they overheard Karl on the phone with Strasser, they determined their old friend must be on some secret mission and he was about to be captured and killed.
When Karl entered the train car to kill Randall, he called for his guards, but they'd been knocked out by Charters and Caldicott. Karl was tied up. Randall took his uniform and Charters and Caldicott took those of the two guards when the train arrived in Munich. After fibbing their way past a suspicious German officer who was waiting for Karl, they managed to get a staff car all to themselves and drove to the Swiss border. Meanwhile, the suspicious officer found and freed Karl and his men, and the enraged Nazis piled into another car and gave chase.
The group arrived at a cable car station which crossed over into the neighboring Switzerland and they locked the attendant in his office. Randall's plan was to send Anna, Dr. Bomasch, Charters and Caldicott across to Switzerland, while he held off the attacking Nazis. He killed the German officer, his chauffeur and a soldier. He then took the second cable car and started across. He exchanged fire with Karl, hitting the Gestapo hauptsturmführer in his leg. After a harrowing leap from his own damaged cable car to another one, Randall successfully joined his friends on the Swiss side.
The wounded Karl simply sat, defeated, inside the cable car control room, powerless to stop them.