After a Momotaro assassin suddenly kills his adoptive father, Shiki vows to avenge him. But first, he must learn to control his newly awakened Oni blood.

Fourteen-year-old Suzuki Iruma has just been abandoned and sold to a demon by his irresponsible parents! Surprisingly, the next thing he knows he's living with the demon who has adopted him as his new grandson, and has been transferred into a school in the demon world where his new "demon" grandfather works as the principal. Thus begins the cowardly Iruma-kun's extraordinary school life among the otherworldly as he faces his true self, takes on challenges, and rises to become someone great.