At a small wharfside restaurant in Nazi-occupied Prague, the washroom attendant, Janoshik, receives a message hidden inside his sandwich. When a drunken Nazi lieutenant named Glasenapp then goes to the washroom, Janoshik attends him, unaware that the lovesick German is going to commit suicide. After Janoshik is summoned upstairs to tend the bar, Glasenapp's friends suddenly realize that he has disappeared. Glasenapp's body is found and the Czech restaurant staff and patrons are all arrested for his murder. When patron Otto Preissinger, a Nazi collaborator and the powerful director of the Bohemian/Moravian Coal Syndicate, protests that a man in his position should not be included among the prisoners, Sergeant Schuler treats him roughly and shoves him back in line. Fortunately for Preissinger, his daughter Milada and her fiancé, Jan Pavel, left the restaurant before the incident. Gestapo Commissioner Richard Rheinhardt is appalled when the coroner determines that Glasenapp was not murdered, but committed suicide, as Preissinger is a friend of Hermann Göring. However, Protektor Kurt Daluege, the highest German authority in Prague, turns the situation to their advantage by making a secret agreement with Rheinhardt to change the coroner's finding to murder, and kill the prisoners so that they can take control of Preissinger's valuable operation. The cellmates become alarmed when Schuler brings them a newspaper report which announces that they will be executed in seventy-two hours unless the murderer is apprehended. When Rheinhardt refuses to see Milada and Jan, the publisher of Prague's major newspaper introduces them to Paul Breda, a Czech hired by the Nazis as a translator and censor, but Paul agrees to help them only after Milada promises to pay him handsomely. Rheinhardt is deaf to Milada's pleas, however, and secretly has her followed by agent Mueller. Unknown to the Gestapo, Paul is a member of the Czech underground, and Janoshik, whose real name is Karel Vokosch, is its current leader. As Janoshik is the only person in communication with the local longshoremen, his arrest has delayed the planned bombing of the Nazi munitions depot at the wharf. In order to free him, Janoshik's comrade Marie concludes that another underground member must give himself up for the murder so the hostages will be released. One man, Joseph, volunteers, but is killed by Nazis before he can turn himself in, so Paul takes his place. That night, Paul informs Milada and Jan that her father will be saved, and despite Jan's distrust, Milada agrees to meet him with the money in the restaurant. Paul and Milada are grabbed by Mueller, but the restaurant is filled with underground members, and Paul kills Mueller when Marie turns out the lights. Later that night, Milada joins the underground but is devastated to learn that Paul is giving his life for her father. In prison, meanwhile, Janoshik suddenly realizes that Glasenapp must have committed suicide, as he had left the man alone in the washroom, and that the Nazis are getting rid of them because of Preissinger. When Janoshik is overheard talking about Glasenapp's purported suicide letter, he is interrogated by Rheinhardt. Rheinhardt fails to see through Janoshik's pose as an ignorant country peasant, and has Schuler and his men take him to the restaurant to find the letter, which Janoshik claims he did not mail because of his arrest. Schuler brutally beats Janoshik, who goes into the washroom to be sick, and escapes out the window into the ocean. With Janoshik free, Paul does not turn himself in, and Jan believes that he has cheated Milada. However, Rheinhardt is shocked when he learns that Karol Vokosch slipped through his grasp, and unsuccessfully attempts to use Preissinger to manipulate Milada into revealing Paul's hiding place. On Janoshik's instructions, the underground distributes leaflets to the public informing them of Glasenapp's suicide and the fact that the prisoners will be executed so the Nazis can control Preissinger's fortune. Paul and Milada go to see Rheinhardt, who forestalls his plan to torture Paul so they can witness the executions of the prisoners. At that moment, innocent-looking fishermen, under the leadership of Janoshik, bomb the munitions depot. When he hears the explosion, Rheinhardt orders the soldiers to use machine guns on the prisoners. Paul and Milada are asked to leave the room when the Protektor comes in, after which Daluege reprimands Rheinhardt for bungling the entire episode. As no one else knows of their secret plan to take over Preissinger's company, Daluege shoots Rheinhardt and makes his death look like a suicide, then promotes Schuler. As he leaves the building, Daluege inquires about Paul and Milada's presence. Paul, thinking quickly, tells him that Rheinhardt wanted him to witness the executions for the newspaper, and Daluege instructs him to report that Rheinhardt has left Prague for a "well-deserved rest in the Fatherland." Stunned by their good fortune, Paul and Milada walk out of Gestapo headquarters.
