FBI agent Ed Renard (Edward G. Robinson) goes on the hunt for a Nazi spy ring bent on subverting the citizenry of America. Beginning with Nazi rabble-rouser Kurt Schneider (Francis Lederer), Renard uses his well-honed instincts to find his way to the ring leader of the espionage campaign, Dr. Karl Kassell (Paul Lukas). With the Nazi secret police rounding up any security leaks and passing them back to Germany, Renard races to extract the information that will bring down the conspiracy.