Reiji returns to his family's abandoned countryside home after eight years, where he is reunited with his childhood friends, sisters Midori and Shiho. As they help him clean the house and revisit their old school, the long-dormant tension between them resurfaces, leading Reiji into passionate and unexpected romantic encounters with both sisters.
After a series of sexually charged encounters with both Shiho and Midori, Reiji is plagued by guilt over his confusion between the two girls. When a heartbroken Midori disappears, Reiji tracks her down to a nostalgic childhood swimming spot where she confesses her long-held fantasies about him. The two reconcile their feelings and agree to a passionate new arrangement as "lovers with benefits."
Reiji's life becomes increasingly complicated when Chisa blackmails him into taking her virginity to prepare her for a visit with her long-distance boyfriend. Despite his resolution to stop being a push-over, he is caught peeping on the Kaji twins, Haruka and Sayaka. The twins capture and bind him, turning the situation into an intense sexual encounter that leaves them all craving more.
Midori attempts to end her relationship with the promiscuous Rei, but she is quickly overwhelmed by a group of girls who insist on sharing him. Despite her initial resistance and attempts to break up, Midori is drawn into their hedonistic lifestyle during a fireworks festival. By the next morning, the boundaries of their relationship have completely dissolved, leading to a chaotic group arrangement as their wild summer begins.