Jim and his mouse friend Gran toll away working as innkeepers. They dream of sailing far away on their own boat. One day, a mysterious fellow appears at the inn and gives them a map. It's the map to Treasure Island. So they decide to make their own boat, and sail away treasure hunting. But it’s not as easy as they think when the run into a group of unsavory pirates led by the dastardly Captain Silver. Silver wants the treasure for himself, and it is up to Jim, Gran, and their new friend Kathy, to beat him to it. 
 
(Source: Discotek)

Jim is working as an innkeeper, together with his friend, the mouse Gran. One day, a mysterious man visits the inn, leaving a map behind. The map shows an island with a treasure. So the duo sets off in their own little boat, trying to fulfil their dreams and leaving the inn far behind. But it seems more people knows of the map, as they cross paths with Captain Silver and his band of wild pirates. Jim, Gran and their new friend Kathy tries to beat them in the treasure hunt that ensues. 
 
(Source: Anime News Network) 
 
Note: A manga adaptation created by Hayao Miyazaki as promotional tie-in for the film.