Fifteen-year-old Mary Ann Markham, and her best friend, Jennifer Martin, decide to host an innocent Halloween party for some schoolmates at the Victorian mansion of Mary Ann's wealthy grandfather. Her live-in writing coach, Art Parker, and his fiancée, Marsha Brown, M.D., have joined the other party attendees in a rather complicated treasure hunt when the game is interrupted by the discovery of a very dead body in a cave.
After Mary Ann and Jennifer are nearly killed in a school bus accident another body is found in the cave, and then two more, but what makes things even more bizarre is the presence of symbols indicative of black magic. While the local sheriff's department seems stymied, Art, Marsha, and the two girls join a local hiking club, thinking maybe its members are somehow involved in the murders. However, instead of finding the murderer or murderers they discover another body in the cave.
Things go from bad to worse, with Mary Ann, Jennifer, Art, and Marsha all in line to be victims before the mystery is solved.