The manga based off the best-selling The Decagon House Murders novel by Yukito Ayatsuji! The members of a particular university's Mystery Club travel to a ten-sided house on a remote island...which just happens to have been the scene of a gristly, and unsolved, mass murder. Armed with details of the murder and keys to the property, they set out to dig deeper...but can they trust the "facts" of the case? And moreover...can they trust each other?
After arriving on the island, the Mystery Club discover mysterious tablets with things like "Detective" and "Murderer" written on them. They initially see it as a tasteless prank, but later that night, the tablet reading "First Victim" ends up on somebody's door, and the group is forced into solving a real mystery...
Meanwhile, on the mainland, Kawaminami and Shimada are getting closer to the secret of the island after receiving letters from Seiji Nakamura, who's supposed to be dead...!!