Gamba to Kawauso no Bouken

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)