There's this powerful guy who decides to give his three peoples, who happen to be Droga, Unne, and Zande gifts before he goes away forever. Zande got the gift of human life, and thought it was a horrible gift so he decided to destroy the world by disrupting the balance of light and dark. In doing so he causes an earthquake near the town of Ur. This earthquack opened up a cave and four adventurous boys decide to explore it. They find the Earth Crystal in the gave and if grants them the power of light to stop Zande, so off they go to save the world.

That's the NES version of the story