"Onmyou Taisenki" is set in a world where the ancient art of Onmyoudou, a form of Japanese esoteric cosmology, is used to combat malevolent spirits known as Genma. The story revolves around Riku Tachibana, a young boy who becomes the chosen wielder of the Byakko, a powerful Onmyoudou artifact shaped like a white tiger. This artifact grants him the ability to summon and control shikigami, spiritual entities that aid in battle against the Genma. Riku is joined by a group of allies, each wielding their own elemental Onmyoudou artifacts, representing the four celestial beasts: Byakko (White Tiger), Suzaku (Vermilion Bird), Genbu (Black Tortoise), and Seiryuu (Azure Dragon).

The narrative begins with Riku discovering his role as the Byakko user after encountering a Genma attack in his hometown. He is guided by Shutendouji, a mysterious and powerful Onmyouji, who introduces him to the broader conflict between humanity and the Genma. As Riku and his allies travel across Japan, they uncover the origins of the Genma and their connection to the Onmyoudou artifacts. The Genma, led by the enigmatic Taiitsu, seek to disrupt the balance between the human and spiritual worlds, aiming to plunge the world into chaos.

Throughout the series, Riku and his companions face numerous challenges, including battles with powerful Genma generals and internal conflicts within their group. Each character’s backstory is explored, revealing their personal motivations and struggles. Riku’s growth as a leader and his deepening understanding of Onmyoudou form a central arc, as he learns to harness the full potential of the Byakko while grappling with the moral complexities of wielding such power.

The plot intensifies as the group uncovers the truth behind Taiitsu’s plans and the ancient prophecy tied to the celestial beasts. The final arc culminates in a climactic battle where Riku and his allies must confront Taiitsu and prevent the destruction of both worlds. The series blends action, mythology, and character-driven drama, offering a narrative that explores themes of destiny, sacrifice, and the balance between light and darkness.