The Adventure of Gamba and the Otter [ガンバとカワウソの冒険]

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.