Namba, Gonda, Tokimitsu, Nishikawa, and Okada, who were a close group of high school friends, hold a small farewell party on their way home from the funeral of their leader, Maiko Matsumoto.
They are excited to see each other again after a long time, but they decide to go to the riverside in the middle of the night to dig up the time capsule that they buried together at the time of their graduation and uncover Matsumoto’s secret.