The Nazis invade Prague in 1939, and inventor Axel Bomasch, who is developing armor plating that is essential for Allied defense, attempts to flee to England with his daughter Anna. Anna is captured, however, and taken to a concentration camp, where her captors interrogate her about Bomasch's location. She refuses to divulge what little information she has and soon becomes friends with Karl Marsen, a teacher imprisoned for his anti-Nazi views. Karl helps her escape, and together they make the dangerous journey to London, where Karl contacts his friend, Dr. John Fredericks. Karl and Fredericks speak in private, and it is revealed that Karl is a Gestapo agent who has gained Anna's confidence in order to follow her as she searches for her father. Karl advises her to place a newspaper ad in the personals section, telling her father that she is in England, and soon she receives a mysterious phone call instructing her to go to Brightbourne to meet a man named Gus Bennett. Anna does not reveal her destination to Karl, but she is followed, nonetheless, as she meets Gus, a brash English intelligence officer disguised as a seaside singer. Anna is reunited with her father, who is now working at Dartland Naval base, but she soon grows weary of the restrictions Gus places on her freedom to protect her father. The two quarrel over Karl, whom Anna wishes to contact but Gus suspects, and Gus's fears are realized when Karl arranges the kidnapping of the Bomaschs. Anna is heartbroken upon learning Karl's true identity and outraged when he threatens to put her in a camp if Bomasch does not work for the Nazis. Meanwhile, Gus has infiltrated the German high command by posing as Major Ulrich Herzog, an engineer. Gus intimates to Captain Prada, the controller, and Admiral Hassinger that he had a love affair with Anna four years earlier and can therefore persuade her to effect Bomasch's cooperation. Karl is jealous when Gus arranges to spend the night with Anna in a hotel, and he foils Gus's plans for escape the next morning by insisting on escorting them on the train to Munich. At the train station, an Englishman named Caldicott, who is leaving Germany with his friend Charters, recognizes Gus as Dickie Randall, with whom he went to school. Gus's Nazi uniform and avoidance of Caldicott makes him suspicious, especially after the train is temporarily stopped, and he and Charters overhear Karl confer with headquarters. They realize from Karl's telephone call that Gus is an undercover agent who is in grave danger now that his identity has been discovered. The timid pair determine to help Gus and stowaway on the train when it leaves. Caldicott and Charters inform Gus of the danger and agree to help him. When the train reaches Munich, Charters and Caldicott aid Gus in tieing up Karl and two guards, and they all escape in a car. They are pursued by Karl and his men to the Swiss border, where Gus holds off the soldiers as Anna and the others escape on an aerial tram. After wounding Karl in the leg, Gus makes a dangerous mid-air leap to the tram bound for Switzerland. He and Anna then finally acknowledge their feelings for one another with an embrace.