After visiting her deceased teacher's grave, Asahi Sena ends up saving a man falling from the stairs. The man, Itsuki Tobari turns out to be her deceased teacher's younger brother. They start to talk about things from the past, about the death of Itsuki's sister and the 11-year-old boy, Sena's best friend from the past, who killed her. Where and how is the boy now? Together, they start to find out what actually happened during that tragic day.