Six months after Nakamura Seiji, the architect and owner of the Decagonal Building mysteriously died in the main building, seven men and women from the University's mystery research group visit the uninhabited Tsunojima Island, where the building is located, for a training camp. Around that time, Kawaminami Takaaki, a former club member, receives a letter from the supposedly dead Nakamura, and teams up with Shimada Kiyoshi in order to investigate the truth behind the mystery surrounding Tsunojima. Meanwhile, at the Decagon, one of the members of the group turns up dead... As a result the remaining members are all in a state of confusion about the culprit and begin to suspect one another, but it will be a week before they can leave the isolated island. Is there a connection between the two murders? (Source: Drama-Otaku) Adapted from the novel "The Decagon House Murders" (十角館の殺人) by Ayatsuji Yukito (綾辻行人).