Prior to the United States entry into World War II, Nazi spies try to steal American military secrets. Among those whose passions are roused is Kurt Schneider who was court-martialed and dishonorably discharged from the US Army. Schneider is not very bright and is easily swayed by the oratory of Dr. Karl Kassel, a prominent physician who is eventually made the head of the Nazi spy ring. When Schneider's contact is arrested in Scotland, the US military asks the FBI to root out the spies. Agent Edward Renard is put in charge of the case and they methodically arrest all who have been spying.