In this world, young people who travel to another world (a fantasy realm) and return are known as “returnees” and often possess extraordinary powers. The protagonist, Akatsuki Ousawa, declined to remain in the fantasy world (Alayzard) after defeating the Demon King and instead returned to Earth — bringing with him the Demon King’s daughter, Miu Ousawa (who must pretend to be his sister). Back on Earth, they enroll in a special institution called BABEL Academy, which trains returnees who retain powers from other worlds.