Asatte no Houkou follows the lives of Karada Iokawa and Shouko Nogami. Karada is a young girl who is about to start junior high school while Shouko is a young woman who has just returned from studying abroad. Shouko also happens to be the former girlfriend of Karada's older brother, Hiro.
On the day of Shouko's return, Karada takes her and Hiro, along with a few friends, to the beach. Shouko becomes annoyed with Hiro for leaving her alone in the United States and intentionally insults Karada's ribbons by calling them childish. This greatly upsets Karada, who dislikes being treated as a child.
Later that same day, Karada is found by Shouko praying at a shrine, wishing to become older. Miraculously, Karada's wish is granted, and she suddenly transforms into a young woman. Meanwhile, Shouko loses her adulthood and regresses to an eleven-year-old.
