Following their battle against the Vertex, YUuna, Fuu, Itsuki, and Karin are joined by a fully recovered Sonoko as the Hero Club resumes its normal activities. However, Yuuna starts to get a strange feeling that someone is missing from the equation. The next day, upon experiencing more deja vu, both Yuuna and Sonoko come to the realization that someone has altered their memories to make them forget all about Tougou, as if she never existed. Upon recalling their true memories of Tougou, who had been taking up a masked hero persona to make up for her sins, the girls are left with the question of where she has disappeared to.
Divine Era, year 298, two years before "Yuuki Yuuna's Chapter", Washio Sumi who was chosen by Shinju-sama, is a hero who protects the world alongside her classmates, Nogi Sonoko and Minowa Gin. It seems like Sonoko and Gin are trouble-makers because they could be idiotic or too energetic. Sumi is excited that she should lead the other two, but...