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Joan Grahame, an English schoolteacher living in Nazi-occupied Denmark, is secretly working with the Danish underground, although Franz Von Brinkmann, the German Gestapo chief, believes that she is loyal to Germany. Angered by the failure of his agents in London to obtain news of a possible British invasion of the Danish coast, Von Brinkmann decides to send Joan to England, warning her that her desertion would endanger the lives of her friends in Denmark. Von Brinkmann first orders Joan to Lisbon, where her story about being held in a German concentration camp is met with suspicion by the British attache, who decides to arrange passage for her to South America rather than London. Soon after, Joan is contacted by George Merrick, a Nazi spy working undercover as a radio operator, who tells her that he will get her to England. After boarding the boat on which Merrick serves as a radio operator, Joan notices that Anthony Lawrence, a drunken Englishman whom she glimpsed in Lisbon, is also onboard. Once at sea, Merrick cables a German submarine to torpedo the boat. As the vessel sinks, Merrick and Joan jump overboard and are joined by Lawrence. Merrick tries to drown Lawrence, but is overpowered by the Englishman. After they are rescued by a British destroyer, Joan sneaks into sick bay to kill Lawrence, fearing that he has learned too much from the delirious Merrick. The wiley Lawrence foils her murder attempt by switching beds with Merrick, and as a result, Merrick dies from an overdose of morphine administered by Joan. Aware that Lawrence knows that she is an enemy spy, Joan is relieved when he is questioned on suspicion of murder. In reality, Lawrence is a British spy who has been trailing Joan. Lawrence follows Joan to London, and when he threatens to turn her over to the police, she confesses rather than endanger the lives of her friends. Convinced that Joan is telling the truth about her relationship with the Germans, Lawrence reveals that he is a secret agent. With Joan's help, Lawrence learns that Rupert Chessman, the owner of a private club, is the head of the enemy espionage ring operating in London. Together, they set a trap for Hoelder, one of Chessman's henchmen, as he installs a wireless beam on a German invasion barge. Joan sneaks onboard the barge, and when Hoelder sees her signalling Lawrence, he shoots her. Hoelder is apprehended, and as Joan dies in Lawrence's arms, the Englishman promises to protect her friends in Denmark. After Lawrence arrests Chessman and his ring, the military authorities, using the invasion barge as a decoy, send the English fleet to meet the Germans. Back in Lisbon, Lawrence is posing as a drunken Englishman once again when he reads the newspaper headlines detailing the latest British naval success.