In Prague, under the iron shadow of the Nazi occupation, the surgeon Dr. František Svoboda [Brian Donlevy]—a member of the Czech resistance—assassinates the brutal “Hangman of Europe”, Reichsprotektor Reinhard Heydrich [Hans Heinrich von Twardowski]. Before Svoboda can slip away, his getaway car is found, and his planned safe house rejects him. A stranger named Mascha [Anna Lee] deliberately misdirects nearby German soldiers searching for him, driving Svoboda to seek shelter at her home. Her father, Professor Stephen Novotny [Walter Brennan], realizes Svoboda must be the assassin, but he is willing to risk everything to hide him.
Because the assassin cannot be found, the Nazi leadership strikes with a brutal standoff: four hundred Czech men are taken as hostages, to be executed in batches of forty unless the killer surrenders or is betrayed by his own people. Professor Novotny becomes one of the enforced prisoners. Mascha pleads that Svoboda surrender so her father might be spared, yet when Svoboda refuses, she goes to the Gestapo to denounce him. The sight of oppressed Czech citizens moves her to plead instead for her father’s release, and the Gestapo arrest Mascha and the rest of the Novotny family, even after bugging their apartment and finding no direct evidence of involvement in the assassination.
Gestapo Inspector Gruber [Alexander Granach] grows convinced that Mascha knows who the assassin is and offers the release of Professor Novotny in exchange for information. Based on staged conversations between Mascha and Svoboda, the Gestapo hierarchy starts to believe that Mascha is having an affair with Svoboda. Yet Gruber persists in gathering intelligence on Svoboda, convinced that truth lies beneath surface appearances. In the background, Emil Czaka [Gene Lockhart], a wealthy local brewer, appears to be a loyal resistance member, but in fact is a German fifth-columnist. When resistance members trick him into revealing his real allegiance, Czaka escapes while many of the others are killed or arrested by the Gestapo.
One wounded resistance leader, Dedic [Jonathan Hale], devises a plan to frame Czaka for the assassination. Acting on planted intelligence, Gruber is persuaded that Svoboda and Mascha are entangled by an affair. Mascha then stages a dramatic scene at a restaurant to bring Czaka to Gestapo headquarters, where she claims that Czaka was the man she saw fleeing after Heydrich’s assassination. The testament of many people supports her story, and Gruber’s suspicions grow only more tangled. When Gruber deduces that Svoboda’s relationship with Mascha has been staged, Svoboda and Jan Horak [Dennis O’Keefe] kill him. Czaka is then framed for this murder as well, and Gestapo agents gun him down.
Although Mascha is told that the remaining hostages will be freed, the Gestapo liquidates them all—including her father, Professor Novotny. Several weeks later, the Nazi Party concludes that Czaka could not have been the killer, but they decide that reopening the case would bring no good. The web of deceit, danger, and sacrifice leaves the family haunted by the choices forced upon them, while the city endures the grim calculus of occupation and resistance.
