Bijo reipu-gari(1987)

Reiko Nozawa, Shizue Sakamoto, Emi Kato, and Takashi Toda went mountain climbing. Takashi was in a love triangle with Emi and Shizue. Shortly after setting up camp by the river, Emi witnesses Takashi and Shizue having sex. She ran into the woods. But suddenly, she is attacked by someone...