Six high school tennis clubmates, including railroad investigator Noriko Hanamura, are going on a day trip to Kawaguchi Lake. However, one of them, Ayako Hirayama, does not show up even after the arranged time for the meet-up has passed. Magazine writer Yumi Tachiki, who planned the trip, calls Ayako but gets no answer, so Noriko and the others, believing Ayako will catch up with them, proceed to their destination. Even though Yumi and Noriko call Ayako many times, she does not answer. Yumi receives an urgent work request after arriving at Tenjozan Park by ropeway, so Noriko and the others return to Tokyo leaving Yumi at the mountain. None of them had any contact with Ayako the entire day. That night, Noriko receives a call from the Yamanashi Prefectural Police informing her that Yumi's body has been found stabbed in Tenjozan Park. One of the ropeway employees happened to witness a mysterious woman wearing a hat and a mask following Yumi down the mountain trail that evening...