Sergio, a young boy living on a remote island with his grandmother, discovers the wounded Lupin III washed ashore after a gunshot and ocean plunge. He aids his grandmother in nursing the injured thief back to health. Simultaneously, his grandmother tends to Fujiko Mine, who resides on the island as a handmaiden for the Nostradamus Sect. She explicitly warns Lupin of Fujiko's cult ties and untrustworthiness. Sergio later accompanies Lupin to the sect's cathedral, leading to Lupin uncovering Fujiko's amnesia and Julia's imprisonment there. When the sect attempts to capture Lupin, Julia, and Sergio, Jigen and Goemon intervene. Sergio escapes alongside Lupin and Julia during the chaos, though Julia is swiftly recaptured. After the conflict, Julia negotiates with Lupin, indirectly acknowledging Sergio by returning Lupin's stolen diamond—on the condition that part of the proceeds funds a new boat for Sergio, fulfilling Lupin's prior consideration. Sergio's involvement is strictly limited to this film's events, without further appearances or development in other Lupin III media.