During the Nazi occupation of Czechoslovakia, surgeon Dr. Franticek Svoboda (Brian Donlevy), a Czech patriot, assassinates the brutal "Hangman of Europe", ''Reichsprotektor'' Reinhard Heydrich (Hans Heinrich von Twardowski), but his getaway car is discovered and therefore his planned safe house must reject him. When a woman whom he does not know, named Mascha (Anna Lee), deliberately misdirects German soldiers close to finding him, he seeks her home as an alternative safe house. This turns out to be the home of her father, history professor Stephen Novotny (Walter Brennan), whom the Nazis have banned from teaching. This plan works. But because the assassin now cannot be found, the Nazi leaders in Prague decide to create an incentive for him to turn himself in or for others to do so. They arrange – with the help of Fifth Column Emil Czaka (Gene Lockhart), a wealthy brewer – for 400 citizens, including Professor Novotny, to be executed, forty at a time, until the assassin is named. Through a complex series of events, however, the resistance manages to frame Czaka himself for the murder, but not before the Nazis have executed many of the hostages.