I watched a certain Lupin III film as a kid at my neighbors’ and never got to learn the title. I really enjoyed that movie and wish to see it again.
This is probably it
After the mishap, Jigen and Goemon arrive in a helicopter and try to help Lupin escape, but Lupin is shot in the arm by Chris and falls into the ocean. The next day, Lupin wakes up on an island and is being nursed back to health by a boy named Sergio and his grandmother. While on the island, he discovers that Fujiko is there as well, but Sergio's grandmother says that she's really a handmaiden of the Nostradamus Sect named Tanya. Lupin follows Fujiko to a cathedral where the sect is located. When he sneaks in, he finds out that Julia is being kept prisoner there and Fujiko has lost her memory. Before he can find out anything else, Lupin is discovered by the sect and they throw them in a cell.