Karada Iokawa, a spirited girl on the cusp of junior high, despises being treated as a child. When her parents pass away, her brother Hiro returns from overseas to care for her. His former flame Shouko Nogami, a composed yet occasionally obstinate woman, trails him to Japan to uncover the reasons behind their breakup. Tensions rise between the two women, culminating in Shouko labeling Karada as immature. Moved by a desire to mature, Karada prays at a shrine, unwittingly triggering a mystical stone to swap their ages. As Karada ages rapidly, Shouko regresses into childhood. Now forced to navigate life in each other's shoes, the two must reconcile not only with each other but also with their own selves.