A group of mystery-loving college students visit Tsunojima Island, the site of an infamous quadruple homicide, and home to architect Seiji Nakamura's unusual Decagon House. Shortly after their arrival, members of the group start turning up murdered, until the fifth day... when the entire Decagon House goes up in flames. Meanwhile, on the mainland, Kawaminami and Shimada discover the truth behind the Tsunojima Island murders from a year ago. But when they visit the grave of Chiori Nakamura, a former mystery club member and Seiji's daughter, they find something that turns the case upside down, and Kawaminami receives word that all the club members on the island have died... All will be revealed in this final volume!