In 1986, Tsunojima Island was home to the Decagonal House, a strange mansion with a decagonal appearance. The genius architect who built the mansion, Nakamura Seiji, died mysteriously in the burnt-down main building, Aoyashiki. Six months later, seven members of the University Mystery Research Club visit Tsunojima, which has become a deserted island, for a training camp. Meanwhile, on the mainland across the sea, Kawaminami Takaaki, a former member of the Mystery Research Club, receives a letter from Nakamura Seiji, who was supposed to be dead. As he continues his investigation, Konan meets a man named Shimada Kiyoshi, and they start working together. Meanwhile, in the Decagonal House, one of the members of the Mystery Research Club is murdered by someone. "Who on earth is the culprit?" The members become suspicious and start to suspect each other... It will be a week before they can leave the isolated island of Tsunoshima. What is the shocking conclusion beyond imagination that arises from these two stories?