For 30 years, thousands have traveled to other worlds. The few that return keep the powers they had in those worlds. Akatsuki Ousawa returns from the world of Alayzard with the defeated Demon King's daughter, Miu. Miu pretends to be Akatsuki's sister to hide her true identity. They join Babel, a school for inter-dimensional travelers, and struggle against the student council to uncover the truth behind the school.
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.