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In 1876. the Molly Maguires, a secret society of immigrant Irish coal miners terrorizing the eastern Pennsylvania anthracite belt, are infiltrated by company detective James McParlan, who poses as fugitive murderer James McKenna.  McParlan rents a room in the home of disabled miner Raines, and courts Raines' ambitious daughter, Mary.

Having secured a job as a miner, the detective ingratiates himself with coworkers by leading the Gaelic football team to victory over its Welsh rival and by beating a brutal policeman.  Furthermore, McParlan allays the suspicions of Molly leader Jack Kehoe by casting the decisive vote to murder a mine superintendent, and by rescuing another Molly during the assassination.

Following the death of Mary's father, Kehoe and McParlan, claiming Raines' right to a burial suit, break into, loot, and set afire the company store.  When Mollies Kehoe and McAndrew attempt to sabotage a mine, however, McParlan alerts Police Chief Davies, and they are arrested.

At the trial the detective's testimony results in the Mollies' conviction and death sentence.  Shocked by McParlan's duplicity, Mary rejects the agent.  While the gallows are being constructed in the jail yard, McParlan confronts the imprisoned Kehoe, then departs for reassignment as head of the Denver Pinkerton Agency.