Sergio, a young boy residing on a remote island with his grandmother, stumbles upon the wounded Lupin III after he is shot and plunges into the ocean. He rescues Lupin, nursing him back to health in their home. His grandmother later exposes Fujiko Mine’s ties to the Nostradamus Sect, a pivotal cult in the film’s narrative, which operates on the island. Sergio guides Lupin through the island’s terrain and aids in infiltrating the sect’s cathedral to rescue the imprisoned Julia Douglas. When cult forces pursue them, Sergio joins Lupin, Julia, and allies in fleeing, though Julia faces recapture soon after. His journey culminates in accompanying the group to the United States to dismantle the cult’s operations, though he remains absent from the final heist and ensuing confrontations.