When Kyoichi's childhood friend Shizuna turns down his affections, his determined little sister Momoko steps in to win his heart. Seeing the bond between her brother and Momoko deepen, Shizuna proposes an unconventional arrangement where she seeks Kyoichi's guidance in matters of intimacy. As the two girls compete for Kyoichi's love, he finds himself entangled in a complex web of emotions, torn between the affections of the siblings.