Former Mystery Club member and K University engineering student Takaaki Kawaminami (Tomoya Oku) receives a letter from Seiji Nakamura (Toru Nakamura). Meanwhile, the club visits Decagon House, an annex of the Blue Mansion on Tsunojima. Takaaki visits Seiji's brother Kojiro and meets the idle Kiyoshi Shimada (Munetaka Aoki). The two hit it off and team up. ©Yukito Ayatsuji/KODANSHA　©NIPPON TV

At the Decagon House on Tsunojima, one Mystery Club member is murdered. With no way off the deserted island for a week, the group spirals into suspicion. Meanwhile, Takaaki (Tomoya Oku) and Kiyoshi (Munetaka Aoki) visit the harbor to investigate the Blue Mansion case, where they spot Masako Yoshikawa (Tamiyo Kusakari), the missing gardener's wife. ©Yukito Ayatsuji/KODANSHA　©NIPPON TV

Takaaki (Tomoya Oku) and Kiyoshi (Munetaka Aoki) keep investigating the Blue Mansion case, but with no leads, they've made no progress. They wonder if the letter was just a prank by the Mystery Club members at the Decagon House, but decide to keep going. Meanwhile, a nightmare unfolds at Tsunojima's Decagon House. ©Yukito Ayatsuji/KODANSHA　©NIPPON TV

Still without a solid lead, Takaaki and Kiyoshi review what they know and arrive at a theory. If they are right, everything fits. To uncover the truth, they visit Kojiro Nakamura again. Could the tragic accident in the Mystery Club room have led to the Blue Mansion massacre? With theories colliding, they decide to head for Tsunojima. But that night… ©Yukito Ayatsuji/KODANSHA　©NIPPON TV

The Decagon House on Tsunojima has fallen into an unimaginable crisis. After hearing about the case from Oita detective Osamu Shimada and facing his friends' deaths, Takaaki is crushed. Ready to distance himself from Kiyoshi Shimada, a word from his landlady (Mari Hamada) makes him investigate again. Where do the two go, and what is Takaaki's final theory? ©Yukito Ayatsuji/KODANSHA　©NIPPON TV