In the 19th-century Pennsylvania coal mines, a secret society known as the Molly Maguires wages a silent war against the oppressive mining companies through acts of sabotage. Among the miners is Jack Kehoe, a compelling and charismatic leader who plays a significant role in orchestrating these acts. To infiltrate and gather evidence against this elusive group, the coal company recruits James McParlan, a detective from the Pinkerton Agency. He arrives in the small mining community under the guise of a common laborer. McParlan's task is risky; he must earn the trust and acceptance of Kehoe and his fellow laborers while maintaining his cover. As McParlan becomes immersed in the miners’ harsh world, he witnesses firsthand the dire working conditions and the despair that fuels their rebellion.

Despite his mission, McParlan finds himself conflicted as he starts to sympathize with the miners’ plight and questions the morality of his own actions. As tensions rise between the miners and their overseers, McParlan is torn between his duty and his growing affinity for the group he is meant to betray. His undercover operation becomes increasingly dangerous as suspicions around him rise, leading to moral dilemmas and a gripping conflict between loyalty and justice. As McParlan pushes deeper into this divide, he navigates through a complex web of trust, deception, and personal conviction, setting the stage for a climax that challenges both his integrity and ideals.