Riho, a working woman, and her boyfriend are moving in together with marriage in mind. While Riho is excited about their happy life ahead, she's troubled by her buttocks, which seem to have a personality of their own and move involuntarily.
They meet a struggling student named Okada at their new place. Concerned about Okada's unhealthy lifestyle, Riho, who works from home, starts bringing him meals and looking after him. Unfulfilled sexually with her boyfriend, Riho finds herself experiencing arousal and desiring Okada's erect penis while preparing lunch at his house.
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