The Kobayakawa twins, Lumi and Luka, are both on the track team and have a crush on Katsuyuki Tohma, an upperclassman. Katsuyuki chooses Luka, leaving Lumi feeling betrayed. While Luka tries to be considerate of her twin's feelings, they both end up stuck in a cave during the celebration for the graduating members of the team. Unfortunately, the cave turns out to be a burial site and the toxic gases take the lives of everyone except the twins. After the traumatic experience, Lumi goes through a significant change and even appears suddenly in Luka's room one day.