"Hi-no-youjin" is a historical fantasy anime set in Japan's Heian period, blending supernatural elements with political intrigue. The narrative centers on Abe no Seimei, a renowned onmyōji (a practitioner of yin-yang divination and magic), who is tasked with maintaining the balance between the human and spirit worlds. As mysterious occurrences and malevolent spirits begin to plague the capital, Seimei investigates the disturbances, uncovering a conspiracy that threatens both realms. His journey intertwines with the lives of key figures, including Minamoto no Hiromasa, a nobleman and skilled musician who becomes Seimei's ally, and various spirits and deities who either aid or hinder their efforts.

The plot delves into the complexities of human ambition, the consequences of meddling with spiritual forces, and the moral ambiguities of power. Seimei's mastery of onmyōdō is tested as he confronts increasingly dangerous adversaries, including rogue onmyōji and vengeful spirits manipulated by hidden forces. The story also explores the tension between tradition and innovation, as characters grapple with the changing dynamics of a society caught between the old ways and emerging influences.

Themes of loyalty, sacrifice, and the interplay between fate and free will are woven throughout the narrative. The characters are shaped by their personal struggles and relationships, with Seimei's enigmatic past and Hiromasa's idealism providing depth to their partnership. The anime balances episodic supernatural investigations with an overarching plot, gradually revealing the true scope of the conspiracy and its ties to the imperial court. The setting is richly detailed, capturing the aesthetics and cultural nuances of the Heian period while incorporating fantastical elements that enhance the story's mystical atmosphere.