Hiroki Katayama arrives in an underwater city and meets the spirit of a mysterious woman at the airport before he has collected his luggage. His uncle settles him into an apartment. That night a strange creature appears in the room — one that tells Hiroki the world is in danger of collapse and that he is involved in preventing it. The creature shows him the other world briefly; when they return, other creatures attack and the small one grants Hiroki the power to destroy them. The next day, school starts and Yumi Asakura — the girl from the airport — is there. The adventure proper begins.