Identical twins Lumi and Luka Kobayakawa share a passion for track and harbor a mutual crush on the charming upperclassman Katsuyuki Tohma, who picks Luka over Lumi. Struggling with her emotions, Luka grapples with the guilt of potentially hurting her sister, while Lumi feels deeply wounded by the perceived betrayal. As they journey to a graduation party for the team's elder members, a sudden storm forces them into a cave that unveils a grim secret—every team member perished due to a mysterious cave odor, leaving only Lumi and Luka alive. The traumatic event transforms Lumi drastically, leading to a shocking occurrence where she inexplicably emerges from the floor of Luka's room, forever altering their bond.