Two years before "Yuuki Yuuna is a Hero." Sumi Washio is only in 6th grade, but she's a hero who was chosen by Shinjyu to save the world. Sumi fulfills her role while cherishing her precious regular life with her classmates, Sonoko Nogi and Gin Minowa. The story moves onto Yuuki Yuuna from Sumi Washio. An incident occurs at the Sanshu Middle School Hero Club who used to have a peaceful everyday life.