Dr. Karl Kassel (Paul Lukas) comes to America to rally support for the Nazism cause among German Americans. He instructs his audience at a German restaurant that the Führer has declared war on the evils of democracy and that, as Germans, they should carry out his wishes. Kurt Schneider (Francis Lederer), an unemployed malcontent, joins the cause and eventually becomes a spy for the group. A letter written by Schneider to a liaison in Scotland is intercepted by a British Military Intelligence officer (James Stephenson), leading to the ring's downfall.