One year has passed after the "Stone Eater", which threatened the whole continent─ "Class Pegasus," after advancing into their last school year, has been preparing for their field trip towards Delta Empire from the West. That year. It was the same year when the world was shaken by a single news from that distant land. The gigantic Tourmaline vein "Labyrinthos" has been discovered─ However, much more than what was initially considered an uncommon ore deposit, there lied a labyrinthine network of Tourmaline caves, dormant under the deep, dark underground. Thus, miners dreaming of making a fortune in one stroke from all over the world swarmed over Delta Empire. It was only the beginning of The Second Tourmaline Rush. "This vein itself... is one enormous 'Dying Wish!'" Pegasus group, guided by afterglow of their lost teacher, pressed on into the ever more labyrinthine "Depths"... "Gather ye, contend in strife, O followers──for the Victor shall be bestowed the Thunder God's honor!" What could be awaiting them at the deepest level?