In the ancient capital of Heian-kyō, the Makai Alchemist Seimei stands as a guardian against the monstrous Horrors that threaten the realm. She operates in a vital partnership with the young man Raikou, the bearer of the Golden Knight Garo armor. As he cannot summon its power alone, Seimei employs her formidable Makai magic to enable his transformations and bolster his strength in combat.

Their duo is frequently joined by a young boy, Kintoki, who dreams of emulating his hero Raikou. This trio forms the core of a dedicated force protecting the city from supernatural dangers, a mission that often pits them against the fallen Makai Alchemist Ashiya Douman, who commands an army of Horrors.

When an ancient cherry blossom tree becomes the vessel for a powerful Horror, Seimei, Raikou, and Kintoki face a new threat. The Horror is awakened by two individuals, Tokimaru and Akira, who seek vengeance upon Emperor Michinaga for the historical exile of their ancestor, Duke Kan. To stop this vengeful plot, Seimei and her allies, including the Makai Knight Fujiwara no Yasusuke, must combine their strengths against the unleashed terror.

Seimei is a master onmyōji, a practitioner of esoteric cosmology and magic. She is also a skilled musician, known to play the biwa.