A man who was once called the world's strongest agent ended up becoming a teacher after his retirement, to train a new generation of agents. After many years of training his disciples, he was killed at the age of 60 by a secret organization's ruse and was reincarnated in another world with his memories intact. Though he was surprised by the existence of magic and the strange species in that world, he quickly adapted to his condition as a newborn and took advantage of it. He acquired special magic and gained a massive amount of strength thanks to his rigorous discipline, in order to reach his goal: Resume his career as a teacher which he left halfway through in his previous life. This is the story of a man, who, based on the memories and the experiences of his previous life, became a teacher who travels through the world with his students.

"Hai to Gensou no Grimgal" draws its inspiration from classic J-RPGs such as Dragon Quest, Final Fantasy, Romancing Saga, Suikoden, Star Ocean, Xenogears, and many others. Those who are familiar with RPGs coming out of Japan when the Playstation 2 was the king of videogame consoles will immediately find themselves on familiar footing. That being said, recently there has been a rise in popularity of series such as Sword Art Online, which also takes place in a world patterned on MMO RPGs. Series such as SAO are defined by characters who are genius fighters and can cut down anyone in their way without much preparation or training. Grimgal purposely seeks to rebuff this idea by introducing characters who must, quite literally, earn their way in the world. Following the development of these characters; their laughter and tears, triumphs and defeats, and perhaps most importantly, their relationship with one another other, is a major aspect of the story.