Jim Nolan, a young man with nothing left to lose, abandons his boarding house room and makes his way to the office of Harry Nilson, recruiter for the Communist Party. Harry interviews Jim and learns his sad life story: he's endured poverty, a lost sister, a drunk and violent dad, a mom who has checked out, and jail time on trumped-up charges.
Harry determines that Jim will be a good candidate and brings him to a house to meet his new posse: Mac (the brains and brawn), Dick (seducer extraordinaire and procurer of funds), and Joy (zealot and punching bag).
Soon, Mac and Jim are hopping trains to reach the apple orchards in Torgas Valley, California (a fictitious location), where disgruntled fruit pickers have just suffered a major cut in wages. Mac and Jim hope to organize a strike that will carry over to the cotton fields in the South. They have the larger goal of spreading the ideals of wealth equality and justice for workers around the country.
They meet Al Anderson, owner of a local lunch wagon and secret Communist sympathizer. Mac collects info from Al about the workers and heads out to the camp with Jim. Mac pretends to have medical training and helps to deliver the grandbaby of a leader called London. London is mighty grateful.
Jim and Mac begin working alongside the men, hoping to convince them to strike. London introduces Mac to another "boss" called Dakin. This guy's not biting, since he's pretty prosperous, but Jim says just the right thing to convince him and his workers come over to the strike.
Mac and Jim hustle to arrange everything the workers will need when they walk out. They visit Al's father, Anderson, who owns a small farm nearby. Mac convinces Anderson to let the workers camp out in one of his empty fields. He also calls in Doc Burton, a sympathetic physician, to handle issues of sanitation and health.
The whole situation in the orchard pops when old Dan, a cantankerous senior worker, falls from his ladder and breaks his hip. The men's anger at their working conditions boils over, and they walk out.
The superintendent of the orchard meets London to figure out the workers' intentions. He offers Mac and London a bribe, but then threatens them when the men refuse to bite. The workers are evicted, so they pack up everything quickly and move to the new camp in Anderson's field.
The leaders of the strike make plans to intercept the scabs—replacement workers brought in to break the strike—at the train station the next day to "persuade" them to join the strike. Mac and Jim step outside the camp to try to win over the police and nearly get themselves lynched.
The dispirited workers march to the train station while surrounded by hostile police forces. When they arrive, the scabs are hesitating in the train yard—until crazy Joy appears and tries to lead them over to the workers. A random vigilante guns Joy down in front of everyone.
Mac brings Joy's corpse back to camp, hoping to spark the men's anger. The next day, Jim and Sam—a young man with serious striking cred—go out with a group of picketers to confront the scabs already working in the orchards. Things get violent, and Jim suffers a gunshot wound to his shoulder.
Discord grows in the camp. Al's lunch wagon has been burned down, and Al himself has been assaulted by vigilantes. Anderson is not amused. Dakin, who has begun to distrust Mac, winds up in a violent confrontation with police, has his trucked burned, and is imprisoned. Mac gives a stirring speech at Joy's funeral.
Anderson's barn—along with his entire crop of apples and his beautiful doggies—goes up in flames. Mac and Jim notice that Doc Burton is missing and suspect that vigilantes have kidnapped him. Mac encourages Sam to burn down landowner Hunter's house in retaliation for the barn incident. And Mac has to "teach a lesson" to a teenager who has been sniping at the workers with a rifle.
Meanwhile, restless Jim takes matters into his own hands. He orders London to prepare a raid on police road barricades, and he bosses Mac around. Mac fears that Jim is becoming a monster and tells him so.
When the barricade run fails, Mac's utterly disgusted with the workers' cowardice. Burke—another "boss" of a group of workers—publicly denounces London for taking bribes (which he is not doing). London breaks Burke's jaw in front of the workers.
Excited by the violence, the men follow London on foot and smash the road barricades. But they are so worked up that they return to camp ready to lynch Mac for calling them cowards. London intervenes.
A heavily armed sheriff appears at the camp with an eviction notice; Anderson has had enough. Mac tells London that the workers will have to stand and fight—and he taps Jim to help him convince the men. Jim can hardly contain his excitement: this is the chance he's been waiting for all along.
A young boy enters the camp to tell Mac and Jim that he's found an injured doctor in a nearby field. The men think it might be Doc Burton, so they run recklessly toward the spot outside the camp. Shots are fired. Mac reaches out toward Jim and discovers that the young man's face has been blown off.
Mac carries Jim's body back to the platform at the camp, where he displays it for all to see. Mac immediately launches into his rote speech to rally the workers, putting his grief to use for the "greater good."