Depicting the story of a shut-in, Kirihara Yukito, after being reincarnated into another world.His peaceful life with his precious caring sister, Sana, suddenly come to an end when a beast appeared and attack them, thus reincarnating him into the “Ever World”. Moreover, knowing that Sana also got reincarnated into the Ever World, for the sake of finding his beloved little sister he begins his journey in another world…
In the fantasy story, the demon king Leonis sealed himself away for 1,000 years in order to prepare for the final battle, but when he woke up, he found himself in the body of a 10-year-old boy. A beautiful young woman named Lisellia from the Holy Sword Academy then says to him, "Why were you locked up in such a place? It's OK now, this big sister will take care of you!" Under Lisellia's care, Leonis is shocked to see that the world has completely changed. Bewildered and confused, Leonis finds himself enrolling at the Holy Sword Academy.