The world known as Terra, is a planet covered almost entirely by water. Living on tiny, isolated islands, the people of this world somehow manage to keep civilization alive. With the need to travel from island to island, people have invented air ships, which come in a variety of shapes and sizes. Life goes on but requires one thing to keep it that way, Quantum Refractors.

Below the surface of Terra there exists an entirely different world. This one is dark, ancient, mysterious, and inhabited by dangerous machines known as Reaverbots. The world above and the world below are linked by Sub-Gates, usually small buildings with ladders or elevators that lead down into the underground. Dangers abound in the subterranean ruins, but they must be explored, for that is where people obtain the highly sought after Refractors.

Those who dare enter the world of the Reaverbots to retrieve the Refractors, are known as Diggers. Diggers keep the world supplied with enough Refractors to keep civilization going. A Digger's means of traveling is of course by air ship, which can take them to just about any ruin in the world. However, air ships also consume the power of the Refractors and so the ravaging of the ruins goes on and on.

Eventually, there came to be people who would seek Refractors for their own profits as well as steal Refractors from other successful Diggers. These people also use air ships to travel from place to place and are called Air Pirates...

The legendary Mother Lode is a rumored treasure sought after by diggers everywhere, and of all sorts. What exactly the treasure may be is the center of much speculation, but many people have their own opinions and dreams on the matter.

One possible and likely resting place for this fabulous treasure is the mysterious Forbidden Island. Far up north, the island is constantly surrounded by an extremely violent storm cloud, keeping most people away, and causing the bravest never to return.

But many years ago, two men went to the island and did return, keeping secret what they saw there. Presently the two men, Von Bleucher and Barrel Casket, ride towards Forbidden Island in a giant air ship called the Sulpher Bottom. Von Bleucher has spent the entirety of his massive fortune building the great air ship, a ship strong enough to face the vicious winds surrounding the island, so that he and Barrel would be able to make a return expedition.

Von Bleucher seeks the Mother Lode on the island, but Barrel is more reluctant to return. 10 years ago his daughter and son-in-law journeyed to the island and like all the rest, never returned. As they approach the island, Barrel asks Von Bleucher one last time if he will not reconsider, but he has come too far to turn back now.

The two of them move to a large conference room where reporters and other members of the press stand waiting to hear what they have to say. As Von Bleucher begins his speech, large panels open on the sides of the Sulpher Bottom, revealing to everyone through large windows, the swirling clouds and lightning surrounding Forbidden Island. They are now very close to it indeed. The conference continues, until there is a disturbance among the people. One of the reporters is not who she claims to be.

Barrel recognizes her as Matilda, his daughter, but cannot believe his eyes. She delivers a mysterious warning to stay away from the island and then makes her escape. Among breaking glass, gunfire from Von Bleucher's guards, and blowing wind, she hops out a shattered window and onto a flying Reaverbot. Everyone is left mystified by this strange occurrence, but a group of air pirates disguised as reporters sense that this marks the presence of a great treasure...

Some distance away, Barrel's granddaughter Roll and her best friend Megaman watch the conference on TV in their own air ship, and are shocked by the event. Roll doesn't understand why her mother (if it was in fact her mother) would act in such a manner, but she and Megaman head toward Forbidden Island to find out what is going on for themselves.

Getting onto Forbidden Island is a challenge in itself, and what is found there is indeed quite a surprise to everyone... Thus begins a grand adventure!