James McParlan is an operative working for the Pinkerton Detective Agency, who is hired by a major coal company to infiltrate and expose an underground terrorist organization, the "Molly Maguires". The Mollies operate within the improverished mining communities of Pennsylvania and are mostly Irish emigrants. McParlan is selected to infiltrate the organization to pose as a new member, but it quickly becomes apparent that the working conditions are truly wretched. The lawman's loyalties become divided between the law and his fellow countrymen.