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.

高校の仲良しグループだった南波、権田、時光、西河、丘田の5人はリーダー的存在だった松元麻衣子の葬式帰りにささやかなお別れ会をする。
久しぶりの再会に話も弾むが、卒業の際仲間で埋めたタイムカプセルを掘り起こして松元の秘密を暴こうと真夜中の河川敷に向かう事になる。