Two worlds exist in balance: the Upper World and the Underworld, separated by two treasures that were never meant to meet. When one of the treasures is removed, the balance collapses, causing the Upper World to sink. Amid the chaos, Bakusai and the other members of the Five — Kairo, K.Nine, Kinen, and Taiitsu — are drawn into the Underworld. There, they discover a world governed by unfamiliar gravity, dangerous air, and secrets far beyond anything they knew.