Fukami, a secretary engaged to company president Kaido, her employer, is invited by his close friends, the Nogis, to a three-day-two-night tour. The tour group is traveling in a luxurious coach to Lake Megami in the Tateshina Plains. On the way, they are disturbed to hear that Shirai, a hostess who was supposed to come along on the tour, has been murdered. And the next day, the body of Hasegawa, another tour member, is floating in Lake Megami...