Seven students, members of their university's mystery club, decide to spend a week-long vacation on Tsunojima Island off the coast of Japan. Six months earlier the owner of the island was brutally murdered alongside his wife and housekeepers, and the case remains unsolved. Soon after their arrival they begin to suspect that one of their members is intending to kill them one at a time, but who? Meanwhile, back on the mainland a former member of the club named Kawaminami receives a letter saying that the death of a girl who died at a club party one year earlier was murder. And the girl in question just happened to be the daughter of the slaughtered island owner. After learning that several other people have received a similar letter, he also begins to suspect that something sinister is happening.
