Boukensha-tachi: Gamba to 7-hiki no Nakama

冒険者たち ガンバと7匹のなかま

When a badly injured mouse staggers off a ship in Tokyo Harbor, he tells the local mice that he has escaped from Devil's Island, where Noroi, the evil white weasel, has crushed the mice with his reign of terror. Tough-guy Gamba rashly promises to help depose Noroi.

(Source: The Anime Encyclopedia)