Gamba, an adventurous mouse, embarks on a journey with his childhood friend Bobo. They gather experienced mice sailors at a harbor. They encounter Chuuta, a mouse who has been injured and in need of help. He appeals for Gamba and Bobo to assist him in defending the island of Yumemishima and its inhabitants from the cruel and wicked weasel Noroi.

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.