During treasure hunting's golden age, Krai and friends aspired to be great adventurers. As they pursued this dream, Krai, realizing it wasn't for him, wished to retire from the perilous life of a treasure hunter.

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.