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 the Mystery Club arrives on Tsunojima Island, they discover mysterious tablets with things like "Detective" and "Murderer" written on them. They initially see it as a tasteless prank, but when the roles written on the tablets start to come true, and the victims begin to pile up, the group realizes their lives will depend on finding the killer... Meanwhile, on the mainland, Kawaminami and Shimada are chasing after the truth behind the "mysterious quadruple homicide"...