At the age of twelve, karada iokawa is an outgoing and dependable girl who dislikes being treated like a kid more than anything else. following her parents' passing away, hiro, her older brother, returns from studying abroad to take care of her. his former girlfriend shouko nogami also arrives in japan with the intention of discovering why he left her. despite their contrasting personalities and attitudes, tensions arise between them, culminating in shouko accusing karada of being childish.
later on, while praying at a shrine to grow up, karada unknowingly makes a wish that will come true. as she ages, shouko, who is nearby, finds herself regressing into childhood once more. with their roles now reversed, the two women must confront each other and themselves as they navigate this new dynamic.