Shijin grows anxious when his girlfriend Nagisa disappears, so he sets out with Gamba, Bobo, Gakusha, Ikasama and Yoisho for the South Island to search for her. Once there, the mice discover a family of Japanese otters endangered by a wild dog named Black. Another thread centers on Chuta, an injured mouse who staggers off a ship at Tokyo Harbor and meets Gamba and Bobo at a party hosted by the sailor mouse Yoisho. Chuta has fled Devil's Island, where an evil ermine named Norio keeps the mice under a cruel rule of fear. Serving as the last chapter of the Gamba story, this adventure movie reached cinemas a decade and a half after the first TV series.