"Heianchō Onigami Kessen" is set in a fictionalized version of Japan's Heian period, blending historical elements with supernatural and action-driven storytelling. The narrative centers on a world where humans and oni (demons) coexist, albeit uneasily, under the rule of the Heian government. The government maintains control through a specialized force known as the Onigami, warriors who wield powerful weapons forged from the remains of defeated oni. These weapons grant their wielders extraordinary abilities but come at a cost, as they risk corrupting the user's mind and soul.

The protagonist, a young and skilled Onigami named Kintarō, is tasked with maintaining order and protecting humanity from rogue oni. Kintarō is driven by a personal vendetta against the oni, stemming from a tragic event in his past that claimed the lives of his family. As he carries out his duties, Kintarō encounters a mysterious oni named Shuten, who challenges his beliefs about the nature of demons and the morality of the Onigami's mission. Shuten, unlike other oni, displays a sense of honor and complexity, raising questions about the true nature of the conflict between humans and demons.

The plot unfolds as Kintarō uncovers a conspiracy within the Heian government, revealing that the line between good and evil is far more blurred than he had imagined. The government's exploitation of oni and the Onigami's role as enforcers of this system come under scrutiny. Kintarō is forced to confront his own biases and the ethical implications of his actions, especially as he forms an uneasy alliance with Shuten to uncover the truth. The series explores themes of power, corruption, and the struggle for coexistence, with Kintarō's journey serving as the emotional core of the story.

Supporting characters include other Onigami warriors, each with their own motivations and struggles, as well as key figures within the Heian government who manipulate events from behind the scenes. The narrative builds toward a climactic confrontation that challenges the established order and forces Kintarō to make difficult choices about his loyalty, his purpose, and his understanding of justice. The series balances intense action sequences with moments of introspection, creating a layered and engaging story that delves into the complexities of its world and characters.