"Roku no Sekai" is a fantasy anime series set in a world divided into six distinct realms, each governed by its own elemental force: fire, water, earth, air, light, and darkness. The narrative centers on a young protagonist, Haruto, who discovers he is the reincarnation of an ancient hero destined to restore balance to the realms. This revelation comes after a catastrophic event shatters the fragile peace between the realms, unleashing chaos and threatening the existence of all six worlds.

Haruto, initially an ordinary boy from the realm of earth, is thrust into a journey of self-discovery and responsibility. Guided by a mysterious mentor, Kael, and joined by a group of companions from different realms, Haruto must gather the six elemental relics scattered across the realms. These relics, when united, have the power to seal the rift causing the imbalance. Each relic is guarded by powerful beings tied to the history and mythology of their respective realms, presenting both physical and moral challenges for Haruto and his allies.

The companions include Lyra, a skilled warrior from the realm of fire with a tragic past; Soren, a cunning mage from the realm of air who hides a secret agenda; Mira, a compassionate healer from the realm of water struggling with her own insecurities; and Ryn, a rogue from the realm of darkness whose loyalty is constantly tested. Together, they navigate the complexities of their world, confronting not only external enemies but also internal conflicts and personal growth.

As the group progresses, they uncover the truth behind the realms' division and the ancient hero's fall, revealing a deeper conspiracy orchestrated by a shadowy figure seeking to exploit the imbalance for ultimate power. The story explores themes of unity, sacrifice, and the consequences of wielding great power, as Haruto and his companions grapple with their roles in shaping the fate of their world. The series culminates in a climactic battle that tests their bonds and resolves, ultimately determining the future of the six realms.