This hard-hitting, socially conscious drama, the sort of story that Warner Bros. made their hallmark in the 1930s, concerns a factory worker named Frank Taylor (Humphrey Bogart), who is convinced that a big promotion is right around the corner for him. However, the promotion goes to a harder-working Polish immigrant named Joe Dombrowski (Henry Brandon). Angry and upset, Frank is approached by members of a secret organization called the Black Legion, who believe in "America for Americans" and want to drive away immigrants and racial minorities through violent means. Wearing black robes, Frank and the other members of the Legion go on a torchlight raid, driving Dombrowski and his family from their home. With Dombrowski gone from the plant, Frank gets the job, which means more money and a higher standard of living for him and his family. But his outlaw activities with the Legion begin taking up more of his time (and his money, as they make a healthy profit selling robes, weapons, and racist geegaws to their membership), which drives a wedge between Frank and his wife Ruth (Erin O'Brien-Moore). Frank begins drinking and starts slapping Ruth around; she leaves him, and Frank takes up with a floozie named Pearl (Helen Flint). Ed (Dick Foran), a good friend of Frank's, sees that his buddy is drinking too much and ruining his life, so he tries to step in and express his concern. His tongue loosened by alcohol, Frank tells Ed about his secret life with the violent Legion; the next morning, Frank is afraid that Ed might inform on him to the police, so he tells the Legion leadership what has happened. They subsequently order Ed to be captured and executed. While Warner Bros. attempted to avoid the wrath of Black Legion and Ku Klux Klan members by stating that all characters and institutions were entirely fictional, Black Legion was still a brave attack on hate groups, given that lynchings were not uncommon in parts of the United States in the mid-1930s.
by Mark Deming synopsis