For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Jim joins the Communist Party and begins working with Mac. The two make their way to Torgas Valley to organize the disgruntled workers.
Act II
Mac and Jim get the strike underway and move the workers to Anderson's farm.
Act III
Anderson's barn is burned, and the strikers are evicted from the property. They're preparing to fight the armed forces that are coming for them.