Dreaming of adventure, city mouse Gavin and his buddy Matthew join a crew of Ship Mice ready to set sail - just as young Chester bursts in, pleading for help. Stowing away on a boat, the gang heads for Dream Island to defend Chester's family of mice against Winston the White Weasel (Jon Lovitz) and his wicked weasel clan. But is their courage enough?

Gamba, a town mouse with a brave spirit decides to go on an adventure to discover the ocean. On his way, he meets a troubled child mouse, Chuta. He says his family and other mice on a nearby island have been killed by a clan of weasels, led by the terrifying Noroi. Gamba promises to save the island, his family and the other mice. He and his friends team up to defeat the evil Noroi and his clan.