In 1986, There was a hotline shop that was owned by a high school girl, Rumi Natsumi who was intelligent. One day Rumi got a call from her high school classmate called Ryo Sugiura Ryo told Rumi that when he masturbates, he turns into a destructive alien tentacled monster. Rumi told Ryo that they should go to a cafe and test this transformation. By testing it, Rumi had to seduce him. It is revealed that Ryo is possessed by a alien entity and transforms whenever sexually aroused. Rumi's twisted sukeban sister Oyuki blackmails Rumi for seducing Ryo to test it. Will Rumi escape Ryo and Oyuki's wrath? Let's see. Let's just go back to December 23th, 1986 in Rumi's house.