The seven members of a college mystery club head to Tsunojima Island, the site of a mysterious murder-homicide and ten-sided house built by the murdered architect Seiji Nakamura. Before long, the students start dying strange deaths, and the survivors find themselves locked in a battle of wits with the murderer...
Meanwhile, back on the mainland, former club member Minamikawa visits the grave of Chiori Nakamura, another former club member who died in a boating accident. While there, she finds something that turns the case on its head...