In a world where the Oni and Momotaro have been locked in conflict for generations, Shiki Ichinose is just a regular high school dropout. That changes…
Shiki learns he's an oni when attacked by Momotaro.
Oni Agency member Naito Mudano retrieves a wounded Shiki and begins evaluating him, determining if he is fit to join the Agency. However, Shiki only wants to avenge his father. Mudano switches to a harsher test, and Shiki goes berserk. In the chaos, Shiki manages to control his Oni blood and is granted admission to Rasetsu Academy where Mudano teaches. Shiki enrolls, vowing to become stronger.
On the first day of school, the new students are thrown into a high-stakes game of “Oni Tag” against Mudano. Losing = expulsion. Shiki picks a fight with classmate Jin Kougasaki, but he can’t get his own Oni powers to work. Bracing for a finishing blow as their teammate Homare Byobugaura suddenly gets between them. She panics, unleashing her “Blood Titan" so the boys have to defend themselves.
After finally activating his powers, Shiki brings down the Blood Titan, but then can’t move afterward. Jin ignores him to charge at Mudano, but is defeated. With time running out, Shiki and Jin team up to snatch the ball right out of Mudano’s hands. Jin briefly considers keeping the win for himself, but doesn't want to owe Shiki. Just then, Mukkun, a tapir-talkie, arrives with urgent news.
Mudano takes the team on a trip to help the Kyoto base. The Momotaro Agency's Tsubakiri Momomiya and Yomogi Momokusa launched an attack on the Oni, leaving dozens of wounded. Mudano entrusts the students to Commander Oiranzaka, heading to battle. While the others help the infirmary, Shiki looks after a little girl. While doing his best to cheer her up, the corpses of her parents are brought in.
The undead attack the students and Shiki is forced to make a decision that will change his mind.
