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Konan Takaaki, a former member of K University’s Mystery Research Club, receives a mysterious letter from Nakamura Seiji. Determined to uncover the truth, he visits Seiji’s younger brother and meets Shimada Kiyoshi, who soon joins him in investigating the mystery.

During a training camp at the Decagon House on Tsunoshima Island, a member of the Mystery Research Club is murdered. As suspicion spreads among the isolated group, Konan and Shimada search for clues related to the earlier Aoyashiki incident.

Konan and Shimada continue their investigation but struggle to uncover any solid leads. While they question whether the mysterious letter might have been a prank, a terrifying incident unfolds at the Decagon House.

Reviewing the clues they have gathered, Konan and Shimada develop a new hypothesis that could explain the tragedy at Aoyashiki. Determined to uncover the truth, they decide to travel to Tsunoshima Island themselves.

As the situation at the Decagon House grows more serious, Konan learns the shocking details of the incident from the police. Devastated by the deaths of his friends, he considers stepping away from the case, but ultimately resolves to continue the investigation and reveal his final theory.