Shinya's life takes a scandalous turn when his brother Shogo introduces his fiancée, Akane. Upon learning that Shogo is reluctant to be intimate with his future wife, Shinya's other sister-in-law, Sayumi, proposes a risky arrangement: Shinya will act as Akane's sexual practice partner. To prepare him, Sayumi provides Shinya with explicit lessons in the school infirmary, leading to a web of secret encounters between Shinya, Akane, and Sayumi.
After Shizuku catches Shinya and Sayumi together, she eventually confronts her own feelings and enters a passionate relationship with him. Simultaneously, Shinya uses a childhood memento to help Akane recover her lost memories, revealing that he was her true love all along rather than Shogo. With the truth unveiled, Akane and Shinya reunite and commit to a future together.