The Molly Maguires were a secret organization of Irish coal miners established in nineteenth century Pennsylvania to fight oppressive mineowners. Led by Jack Kehoe (Sean Connery), they plant dynamite to destroy plant shafts and equipment. As character James McParlan, Richard Harris portrays real life Pinkerton Detective James McParland who was employed to infiltrate the Mollies.
Kehoe and McParlan are working class immigrants from Ireland with essentially the same aspiration - advancement in the new society to which they have come. McParlan coldly betrays the group whose leader he has befriended and Kehoe kills to advance his cause. McParlan also develops a romantic interest in Mary Raines, but she ends up offended by his treachery. Awaiting execution, Kehoe tells his erstwhile ally that no punishment short of hell can redeem his treachery; Detective McParlan retorts that in that case, "See you in hell."