Edel, a boy who fell into the demon world as an infant, was fortunately picked up and raised by an old man. However, when he turned twelve, his adoptive grandfather passed away. Following his grandfather's will, Edel went to the human world to meet his grandfather's acquaintance, who turned out to be the renowned hero Maribel, one of the Four Heroes. Thanks to Maribel, Edel began attending a hero school. However, he was unaware that the demon world where he grew up was an incredibly harsh environment compared to the human world. He also didn't know that his adoptive grandfather was a legendary great sage revered by the Four Heroes, and that he had been subjected to a Spartan education in the harsh demon world, resulting in him possessing extraordinary powers. 'Huh? I used to encounter Bahamut all the time in the demon world, and even kept it as a pet.' 'Bahamut as a pet!? That's an S-rank monster! ...Teacher, you've left behind an incredible child...' --It seems the demon world-raised boy found the human world far too easy.