In the middle of the 15th century, there was a legend about the King's Heart in the richest adventure kingdom in the Central Continent - if you find the King's Heart, you can become the king.

In order to study the mysterious substances on the seabed, he followed his uncle, the geologist "Ironstone", dived deep into the Mariana Trench, accidentally was involved in the cave maze, came to the huge unknown world and discovered ancient creatures, and started an exploration of the undersea geocenter.