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

One possible and perhaps even likely resting place for this fabulous treasure is the mysterious Forbidden Island. Far to the north, the island is surrounded by extremely violent storm clouds. The perpetual turmoil keeps most people away, and causes the bravest never to return. Many years ago however, two men went to the island and did return, keeping secret what they saw there.

Now, Werner Von Bluecher and Barrell Caskett ride towards Forbidden Island, ready at last for a return expedition. To make it happen, Von Bluecher spent the entirety of his massive fortune to build an equally massive airship, the Sulpher Bottom, a craft strong enough to withstand the vicious winds around the island.

Von Bluecher seeks the Mother Lode, but Barrell is more reluctant to return to the island. 10 years ago his daughter and son-in-law journeyed to the island and like all the rest, never returned. As the Sulpher Bottom approaches the island, Barrell asks Von Bluecher one last time if he won't reconsider, but Von Bluecher has come too far to turn back now.

The two of them move to the ship's large conference room where members of the press stand waiting in anticipation. As Von Bluecher begins his speech, large shutters open on the side of the Sulpher Bottom, revealing to everyone the swirling clouds and lightning surrounding Forbidden Island. They are very close indeed. The conference continues, until a disturbance breaks out among the people. One of the reporters is not who she claims to be.

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

Elsewhere on Terra, Barrell's granddaughter Roll and her best friend MegaMan watch the conference on TV in their own airship. They too are shocked by the event. Roll can't understand why her mother—if it was in fact her mother—would act in such a manner. She and MegaMan fire up the engines and head toward Forbidden Island to find out for themselves what's going on.

Getting onto Forbidden Island is a challenge in itself, and what waits to be found there will indeed be quite a surprise to everyone... So begins a grand adventure spanning all of Terra, and beyond!