In a world covered almost entirely in water, there exist small islands that hide huge treasures. And where there's treasure, you can find treasure hunters called Diggers, who live for the thrill of the hunt...

Barrel Casket was once one of the greatest Diggers, but he has now retired. He now cares for his young granddaughter, Roll, whose parents disappeared while on a dig. He had also discovered a strange baby boy in some ruins one time, who he also took under his care alongside Roll.

He named the baby boy Rock Voulnut, and Rock grew up to be an expert Digger, sometimes known by his nickname, MegaMan.

While out exploring one day, Rock, Roll, and Barrel's airship, the Flutter, crash lands on an island named Kattelox. As they explore Kattelox in hopes of fixing their ship, they hear word of a huge treasure, the legendary Mother Lode, that is buried somewhere on the island.

Rock, Roll, and Barrel decide to look for the treasure, but at the same time, they're going to get a little competition from a pirate family named the Bonnes...