Long ago, demon-like creatures known as Oni slaughtered humans and caused destruction purely for their own amusement. A solitary man named Momotarou stood against these horrific, nearly invincible monsters, yet the war against them was only just beginning. Thousands of years later, the hot-tempered gun enthusiast Shiki Ichinose lives a carefree existence. However, his life changes completely after he is expelled from school. Shiki is suddenly attacked by a man and is saved by his adoptive father, Tsuyoshi. As the pair flee, Tsuyoshi reveals that Shiki is actually a descendant of the Oni, while both he and their pursuer belong to the Momotarou lineage. The two Momotarou soon engage in a fierce battle, and Tsuyoshi sacrifices himself to protect Shiki. Devastated, Shiki awakens to his Oni blood, which 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.