Saito is a ronin (students who have failed the university entrance exam and are preparing to retake it), his neighbor is Sumire, a beautiful and kind woman who always helps and cooks for him. Saito has had feelings for Sumire for a long time but she is happily married to her husband, so he has no other intentions. The walls are quite thin, so every time Sumire and her husband have sex, Saito can hear everything clearly, making him even more jealous of Sumire's husband. Throughout the time, he always thinks about Sumire, and whenever he has free time, he masturbates thinking about her. One day, while in his room, he hears Sumire's high-pitched scream. He rushes out and Sumire unexpectedly hugs him. It turns out there was a cockroach in the house, Saito was about to leave but Sumire asks him to stay and help her get rid of it. While searching for the cockroach, Sumire keeps flaunting her round buttocks in front of him, her panties completely exposed, causing Saito to be unable to resist anymore. He then honestly tells her that he wants to have sex with her. Hearing Sumire's answer, Saito can't believe his ears. She agrees to have sex with him! And their twisted relationship begins from there. Whenever Saito feels 'in the mood', Sumire comes to 'take care' of him so that he can focus on his studies...