10 years ago, five girls were captured and held hostage by a criminal in a mysterious house on Rougetsu Island, located south of Honshu.
They were eventually rescued by Choushiro Kirishima, a detective pursuing the criminal. Several years after the incident, two of the girls, Marie Shinomiya and Tomoe Nanamura, died mysteriously. The three remaining girls, Ruka Minazuki, Misaki Asou, and Madoka Tsukimori, all now 17 years old, return to the island to recover their lost memories and find out more about their kidnapping. Choushiro follows the girls at the request of Ruka's mother, Sayaka Minazuki.