After visiting her college-aged brother Takashi, a devoted older sister discovers his hidden fetish for anal pleasure through a magazine. Driven by a desire to satisfy her brother's fantasies, she experiments on herself before offering her virginity to him. Following a failed date for Takashi, the two succumb to their mutual desires, leading the sister to demand that Takashi return home so she can exclusively serve his every whim.
After a session of intense roleplay and anal training with his older sister, Takashi is surprised by a visit from Yumi, who confesses her own secret lewdness. Despite her lack of experience, Yumi's bold request to join their arrangement leads to a passionate encounter. The two women eventually agree to share Takashi, culminating in a hedonistic three-way experience.
Takashi reunites with his cousin Mizuho, who reveals a long-hidden obsession with him despite being married. The two engage in a passionate affair, fulfilling Mizuho's dream of becoming pregnant by him. Months later, after discovering she is expecting a girl, Mizuho insists on continuing their secret relationship until she conceives a son.
Riko and a rival compete to seduce Takashi through a series of provocative displays, eventually collaborating to ensure they both conceive his children. While the sexual encounters are successful, Takashi's academic performance suffers, leading him to fail his college year. In the aftermath, the women encourage him to drop out of school entirely to dedicate his life to supporting them and their future offspring.