The film, which stars Bruce Cabot, Marguerite Churchill, Ward Bond, and Crawford Weaver,[1] is a fictionalized story about the real-life Ku Klux Klan splinter group called the Black Legion of the 1930s.

Frank and Slim successfully infiltrate the group by undergoing an initiation ceremony in which masked members in long robes blindfold Frank and Slim and hold guns to their heads before giving them each a bullet as a token of their membership.

When Frank tries to convey his sympathy to Nancy, she forces him to confess his membership in the legion, and vows never to speak to him again.

It is then revealed that McCollum was the leader of the legion, after which he and his assistant try to escape, but are burned to death when their car overturns.

Later, in the Post Office Department in Washington, D.C., the chief inspector congratulates Frank and Slim for their work, but warns them that Americans are a nation of "joiners", and as such are susceptible to organizations such as the Ku Klux Klan and the Hooded Legion, which they join believing they are being patriotic, when in fact they are placing themselves in the hands of racketeers who operate the legions solely for their own benefit.