In 1876, a secret society of immigrant Irish coal miners called the Molly Maguires terrorize the mines in eastern Pennsylvania. The group is infiltrated by company detective James McParlan, who gets a job as a miner and courts a local girl. When some of the Mollies attempt to sabotage a mine, McParlan alerts the police. McParlan's girlfriend rejects him for his duplicity and his testimony results in conviction and death sentences for the Mollies'.