During a birthday celebration for a prominent politician, Zenigata probes a counterfeit Lupin calling card. Lupin materializes at the event, only to be interrupted by masked assailants unleashing chaos with a hail of bullets. Zenigata pursues Lupin through the mayhem, finally cornering him in the mansion's courtyard. Just as the tension peaks, a mysterious hand wielding a Walther P-38 shoots Zenigata. Recognizing the gun as his own, Lupin embarks on an investigation. The attackers are revealed to hail from the clandestine Island of Assassins, shielded by complicit government figures for their own agendas. As Lupin, Goemon, Jigen, and Fujiko delve into the island's enigmatic depths, they become entangled in a divisive conspiracy. Amidst the turmoil, the film delves into Lupin's untold history before his partnership with Jigen.