Tekkamen, alias Iron Mask, manifests as an enigmatic captive confined within a solitary island mansion. Shackled to a wall and obscured by an iron mask, his presence fuels intrigue about his identity and imprisonment. The mansion's owner lures Fujiko Mine by invoking Lupin III's name, subsequently attracting associates Goemon Ishikawa and Daisuke Jigen. Tekkamen's true persona emerges as Goemon Ishikawa XIII, a recurring ally of Lupin, following intense confrontations—including a lethal duel between Jigen and Goemon. This deception stems from Lupin III orchestrating the captivity and masking to engineer a surprise reunion for his companions after five years apart. The gathering spirals into chaos with Inspector Zenigata and police forces descending upon the island.