Grandpa Mori must return to Japan and present a ceremonial dagger he won fifty years ago to the winner of a ninja tournament, and he asks his grandsons Rocky, Colt and Tum Tum to accompany him. The dagger, Grandpa confides, is also the key to a secret cave of gold, which is why his nemesis of a half century ago and a trio of spaced-out heavy metal rockers are in hot pursuit.