After visiting her deceased teacher's grave, a guy suddenly fell off the staircase where Asahi Sena ended up saving him. That guy, Itsuki Tobari, turned out to be her deceased teacher's younger brother. They started to talk about things from the past, about the death of Itsuki's sister and the 11-year-old boy who killed her, who was Sena's best friend. What happened to the boy who killed her? Together, they start to find out what actually happened during that tragic day.