The story itself is told for maximum mass comprehension. Based on articles by Leon G. Turrou, former G-man, it is an adaptation of the spy trials of 1937 which resulted in the conviction of four persons.
Its thesis is that espionage directed from Berlin is tied up with the German-American Bunds, their rallies and summer camps and general parading around in uniforms. The German goal is destruction of democracy.
The cast numbers a fine collection of scar-faced Gestapo agents, guys with crew haircuts and assorted livid sneerers.
Edward G. Robinson comes in very late in the film. Paul Lukas carries through as the Bund leader who finally falls out with the Gestapo. The missing motivation, anti-Semitism, is the one thing not named and ticketed.