After Shijin's concern on the absence of his girlfriend Nagisa, Gamba, Bobo, Gakusha, Ikasama, Shijin, and Yoisho travel to the South Island looking for her, only to find out about a family of Japanese otters, endangered by a wild dog named Black. When Chuta, a badly injured mouse, staggers off a ship in Tokyo Harbor, he bumps into out-of-towners Gamba and Bobo, who are at a party being given by Yoisho, the sailor-mouse. Chuta tells the local mice that he has escaped from Devil's Island, where Norio the evil ermine has crushed the mice with his reign of terror.

(Source: Anime News Network)