(Wiki Article of Movie)
The movie has a slow start, but once it finds its stride, it's a pretty compelling watch. Though be warned, it is a product of its time and has some brow-raising levels of sexism.
It's fairly chilling to watch, as it's astonishingly similar to the environment the US is in today (especially after reading the propublica expose on the mole that infiltrated right-wing militias) and it becomes painfully clear how this old and tired playbook is still being used, almost without changes, to dramatic effect.
As the title says, the movie is based on a true story, in this case, the Black Legion was an actual group; a more extreme and violent offshoot of the KKK, if you can believe it. (The wikipedia article basically spoils the movie, so you may want to read it afterward).
What this movie depicts, of the motivations of the people involved, of the system itself, is something we could really use a modern rendition of.