Sanshu Middle School's Hero Club—consisting of Yuuna Yuuki, Karin Miyoshi, and siblings Fuu and Itsuki Inubouzaki—is back in full swing, helping those in need wherever they can after fulfilling their destiny after the events of Yuuki Yuuna wa Yuusha de Aru. They've also recruited a new member, Sonoko Nogi, a former hero. However, they gradually find that Mimori Tougou, who should be among them, is absent; all traces of her presence have vanished, save for the girls' recollections.