When a Nazi officer is assassinated, Czech patriots band together to protect his killer.

In Prague, Czechoslovakia, surgeon Franticek Svoboda is pursued by the Blackshirts, a division of Nazi police, and is assisted in evading them by Mascha Novotny, a passerby who misdirects the Blackshirts. After news comes that the Nazis' head executioner, known as the "Reichsprotector," has been assassinated, Nazi officials close all businesses early. Franticek is unable to find a room for the night because he is out after curfew, so, having gotten her address from a flower shop, he appears at Mascha's door just after her fiancé, Jan Horok, leaves. Mascha realizes that he is the assassin and reluctantly lets him in, pretending that he is "Karel Vanyek," a stranger she met at a symphony. Mascha's father, Professor Stepan Novotny, invites Franticek to spend the night as their guest because he is sympathetic to the underground movement, for which Franticek is working. That same night, the taxi driver who delivered Franticek to the scene of the crime is questioned at Gestapo headquarters and commits suicide rather than face torture. Mrs. Dvorak, a vegetable seller who was seen speaking with Mascha is also questioned and beaten by the Gestapo. Unable to identify the killer, Chief of Gestapo Kurt Haas orders that 400 Czechoslovakian civilians be arrested and held until the killer surrenders. Stepan is among those arrested, but he goes willingly, as he knows the importance of the resistance movement. Franticek suffers a guilty conscience because of the widespread response to his act, but his co-conspirators urge him not to surrender as they feel his struggle is a symbol for all Czechoslovakians. Distraught over her father's arrest, Mascha begs Franticek to give up, but he refuses. Mascha attempts to turn him in to the Gestapo, but meets with resistance from the Underground. By the time she arrives at Gestapo headquarters, Mascha grasps the importance of Franticek's act, and therefore only begs for Stepan's life. Inspector Ritter consults with Inspector Alois Gruber and Haas, who suspect that she is the woman who misdirected the Blackshirts, but they release her. Mascha then goes to see Jan, who is distraught because he has been interrogated all day by the Nazis and thinks Mascha has been unfaithful with Franticek. Mascha allays his fears without revealing the true nature of her involvement with Franticek. As she leaves Jan's apartment, Mascha is arrested by the Gestapo, and they put her in a cell with Mrs. Dvorak. One by one, everyone acquainted with the Novotnys is interviewed, and all remain faithful to the story about "Vanyek," although the Gestapo has confirmed that no one by that name exists. When Gestapo police intercept a note and roses intended for Mascha from "Vanyek," they release Mascha, who then is visited by Franticek. Knowing that Mascha's apartment is bugged, Franticek gives Mascha written lines to repeat. While revealing his true identity to her, Franticek leads the Gestapo police to believe that he is merely a suitor. Haas is convinced of Franticek and Mascha's innocence and calls off his investigation, but Gruber remains suspicious. Franticek and Dedic, the leader of the underground movement, hatch a plan to free the hostages without betraying Franticek. To this end, they invite Czech beer brewer and Nazi informant Emil Czaka to lunch. Czaka suspects it is a set-up and informs Gruber of the appointment. During the lunch, Czaka unwittingly confirms his affiliation with the Nazis, and the underground members try to take him hostage. When Czaka attempts to escape, Gruber's police shoot members of the underground and take others hostage. Dedic escapes to Franticek's apartment with a bullet in his lung, while his captive partners refuse to divulge their secrets to the brutal Gestapo. A taxi driver, meanwhile, informs the Gestapo about the bleeding man he dropped near Franticek's apartment, and Gruber and the police converge on the apartment. Gruber finds Franticek and Mascha in an apparently compromising situation, but fails to find Dedic. However, Gruber shames Mascha in front of Jan, who then goes on a drinking spree with Gruber. The next day, Gruber suddenly recalls a detail from the night before and realizes that he was set up. Jan tries to stop him from leaving, but Gruber knocks him unconscious. The Gestapo, meanwhile, has begun indiscriminate executions of the hostages in groups of forty. Under the watchful eyes of the Gestapo, Mascha and Franticek go to Czaka's favorite restaurant, where Mascha pretends to recall Czaka as the man she saw running from the scene of the murder. Czaka is at first confident of his alibi, but every person then interviewed supports Mascha's story. Czaka's final hope lies with Gruber, with whom he spoke on the day of the murder. Gruber, however, is murdered by Franticek and Jan. When later questioned by the Gestapo about Gruber's disappearance, Jan testifies that Gruber spent the night carousing with him, but then left in the morning. Czaka admits that he was home in the morning, and is shocked when his butler lies that Gruber visited him. Gestapo police search Czaka's house and find Gruber's calling card, a gun whose bullets match those used to kill the executioner, a train time table, and Gruber's dead body. Czaka is arrested as the assassin and the hostages are released. As Czaka is being driven to Gestapo headquarters, the inspector stops the car and releases him, then shoots him in the back as he runs to a church. Later, an official report from Berlin affirms that Czaka was not the assassin, but declares that as "the sharpest terror failed to force the people to denounce the real assassin," they must "save the face of the German occupational authority and choose the lesser evil by accepting Czaka as the assassin and thus close the case."