The bloodlines of "Oni" and "Momotaro" have been passed down among certain humans for generations. Long ago, the Oni, aware of their own ferocity, lived in seclusion. However, their peace was shattered by an invasion led by Momotaro. Over thousands of years, these two factions formed the "Momotaro Agency" and the "Oni Agency," respectively, and have been locked in conflict ever since. The protagonist, Shiki Ichinose, suddenly learns of his Oni lineage following an unexpected attack by Momotaro. This revelation sets Shiki on a path to discover the destiny that lies within his blood — a meeting with the Oni dwelling within him.

In Nerima, Tokyo, the Oni and Momotaro clash once again.