Makiko, a college student, has a younger sister, Yumi, who works at a snack bar. Yumi, who lives a free-spirited life, often stays at Makiko's apartment, and when she gets drunk, she causes financial trouble. One day, on the way home from a party, Makiko and her friend went to a restaurant where Yumi was, and they got extremely drunk and fell asleep. Yukio, a regular customer sent by her, tried to take off her clothes and put her on the bed, but when he saw her naked big breasts, he got horny and raped her roughly. The relationship between the two continued, but in reality, Yumi also had a relationship with Yukio. Makiko, the owner of the girlfriend's big breasts, is subject to Yumi's cruel revenge...