Two years before the events of "Yuki Yuna is a Hero." Washio Sumi, a sixth grader, has been chosen by the Shinju as a Hero to protect the world. Together with her classmates Nogi Sonoko and Mimori Gin, Sumi carries out her duties while cherishing their irreplaceable daily lives. And so, the story shifts from Washio Sumi to Yuki Yuna. What incident will befall the peaceful daily life of the Sanshu Middle School Hero Club?