Long ago, demon-like creatures known as Oni butchered humans and brought on destruction solely for their own delight. A lone man named Momotarou opposed these gruesome, near-invincible monsters, but the war against them was just beginning.

Thousands of years in the future, temperamental gun enthusiast Shiki Ichinose leads a carefree life. However, after his expulsion from school, everything changes. Shiki is suddenly attacked by a man and is rescued by his adoptive father, Tsuyoshi. As the two flee, Tsuyoshi reveals that Shiki is actually a descendant of the Oni, while both he and their pursuer are from the Momotarou lineage.

The two Momotarou soon clash in a fierce battle, and Tsuyoshi sacrifices himself to protect Shiki. Devastated, Shiki awakens to his Oni blood that grants him immense strength at the cost of going berserk. Although the enemy escapes, Naito Mudano, an Oni teacher at Rasetsu Academy, takes an interest in Shiki. Under Mudano's guidance, Shiki must learn to control the Oni blood—if he ever hopes to avenge his father.