Edel, a boy who fell into the Demon World as a baby, was luckily picked up and raised by an old man. But when he turned twelve, his adoptive grandfather passed away. Following his grandfather's will, he went to see his grandfather's acquaintance in the human world—the hero Maribel, one of the Four Heroes. Thanks to Maribel, Edel attends a hero school. However, he didn't know that the Demon World where he grew up was an incredibly harsh environment compared to the human world; that his adoptive grandfather was a legendary great sage revered as a "teacher" by the Four Heroes; and that he had acquired extraordinary powers thanks to the harsh Spartan education in the Demon World. "Huh? I encountered Bahamut all the time in the Demon World? I even kept it as a pet." "A Bahamut as a pet!? That's an ultra-S-class monster! ...Teacher, you've left us with an incredible child..." It seems the human world is too easy for someone raised in the Demon World.