Hector Corbett Finnegan, celebrated publicly as a "modern-day Robin Hood," uses his world-renowned thief status to steal from perceived deserving targets and aid fellow thieves in need. This reputation prompts multiple criminal groups, including Lupin III's crew, to storm Dorrente's maximum-security prison, El Guille, aiming to rescue him from execution. Each of Lupin's team pursues personal motives: Jigen repays a debt for Finnegan's past aid, Goemon reveres his perceived nobility, and Lupin craves both the challenge of surpassing him and claiming his treasure. Fujiko manipulates rival thieves into simultaneous assaults on the prison, planning to exploit one for the treasure.

Behind his heroic facade, Finnegan orchestrates an elaborate deception. He colludes with prison warden Lorensa, transforming El Guille into a front for illegal arms trafficking and black market dealings—including selling captured thieves. He funds these operations with his stolen fortune and holds the King of Dorrente hostage via a bomb implanted in the monarch's neck, controlling its detonator. When Lupin's crew reaches him, Finnegan refuses escape. He betrays them, capturing Jigen after using pickpocket skills to swap Jigen's magnum revolver for a toy. This act fractures the group, leaving them imperiled within the prison walls.

Finnegan's ultimate scheme involves seizing Dorrente's monarchy. He undergoes plastic surgery to resemble the presumed-dead Prince Fio (later revealed as the swordsman Verte) and plans to assume the throne after faking his own death as Finnegan. He conceals the surgery by wearing a mask of his original face. During his confrontation with Lupin and Zenigata, he activates the prison's emergency mechanism—crushing internal structures with monoliths—as a diversion. Finnegan then pursues Verte to a coastal island for a final duel, intending to eliminate the prince and secure his impersonation. Verte, trained by Goemon, defeats him and rescues Lorensa, thwarting the usurpation. Finnegan is neutralized, though his treasure is later divided among the thieves.

Personality-wise, Finnegan exhibits calculated manipulation, exploiting loyalties like Jigen's sense of obligation to advance his ambitions. Vanity drives his surgical transformation, while egotism surfaces in his dismissive attitude toward adversaries like Lupin. He remains ruthlessly focused on consolidating power, showing no remorse for betrayals or endangering the kingdom.