Acording to Square-Enix
Many years ago, on a floating continent hovering high above the surface of an unnamed planet, a technologically advanced civilization sought to harness the power of the four elemental crystals of light. They did not realize that they could not hope to control such fundamental forces of nature. This power of light would have consumed the world itself had the light crystals not had their natural counterparts: the four dark elemental crystals. Disturbed by the sudden interruption of the careful balance of the light and the dark, four warriors were granted the power of the dark crystals in order to recontain the power of the light crystals. These so-called Dark Warriors succeeded in their quest, and restored harmony to the world. But their victory came too late to save the doomed civilization that had foolishly tried to harness the power of the crystals to begin with. Their once-proud culture was reduced to ruin, though their floating continent remained, a reminder of what had come before. And on that very continent, the circle of Gurgans, a race of blind soothsayers and fortune-tellers, predicts that eventually things will come full circle. Just as the power of light can be abused, so too can the power of darkness. And when that occurs, then the crystals of the light will need to call forth their own champions to restore balance to the world....
One day, an earthquake opens up a previously hidden cavern in Altar Cave near the village of Ur on the floating continent. Four orphaned youths under the care of Topapa, the village elder, go exploring and come across a crystal of light. The crystal grants them a portion of its power, and instructs them to go forth and restore balance to the world. Not knowing what to make of the crystal's pronouncements, but nonetheless recognizing the importance of its words, the four inform their adoptive family of their mission and set out to explore and bring back balance to the world.