Students from a university mystery club decide to visit an island which was the site of a grisly multiple murder the year before. Predictably, they get picked off one by one by an unseen murderer. Is there a madman on the loose? What connection is there to the earlier murders? The answer is a bombshell revelation which few readers will see coming.

The Decagon House Murders is a milestone in the history of detective fiction. Published in 1987, it is credited with launching the shinhonkaku movement which restored Golden Age style plotting and fair-play clues to the Japanese mystery scene, which had been dominated by the social school of mystery for several decades. It is also said to have influenced the development of the wildly popular anime movement.