A pair of researchers stays behind on their dying planet of Eltria with their two daughters, Amitie and Kirie, in hopes of finding a way to revive the planet. But when the husband Granz falls ill it seems their dream of reviving the planet will die. Against her older sister's wishes, Kirie sets off with her childhood friend Iris to seek help from a distant alternate world. They arrive in Japan on Earth to search for the key to their planet's regeneration. There, they meet Nanoha, Fate, and Hayate.
Summary taken from: MyAnimeList