After fulfilling their fateful duty in Yuuki Yuuna wa Yuusha de Aru, the Hero Club of Sanshu Middle School, led by Yuuna Yuuki, and including Karin Miyoshi, and the Inubouzaki siblings - Fuu and Itsuki - have resumed their community work. They are joined by new member Sonoko Nogi, but Tougou's mysterious disappearance leaves them troubled. Though they have no leads on Tougou's location, the girls regain their transforming abilities and commence a frantic search. What they ultimately uncover is beyond their wildest expectations, and they soon find themselves facing the dire consequences of their actions, which irreversibly alter their lives.