Sergio, a youth living with his grandmother on a secluded island, stumbles upon the wounded Lupin III washed ashore. Together, they nurse the thief back to health. His grandmother discloses that Fujiko Mine, discovered elsewhere on the island, has ties to the Nostradamus Sect but suffers from fragmented memories.

While Lupin probes the sect’s cathedral, Sergio stays behind under his grandmother’s watch. The boy’s location remains unclear during Lupin’s subsequent capture by the cult, but he later aids Lupin, Julia, Jigen, and Goemon in their escape attempt—a struggle that sees Julia seized again.

After the conflict concludes, Julia brokers a deal with Lupin, directing a fraction of his diamond heist earnings toward replacing Sergio’s damaged boat—a loss hinted to have occurred earlier, aligning with Lupin’s unspoken intent to rectify the boy’s ruined vessel.