Kenji, tormented daily by his voluptuous, big-breasted sister-in-law Yumi, discovers one day that his brother is away on a business trip and receives information that Yumi is cheating. Rushing to the scene, Kenji catches Yumi pleasuring herself and is overwhelmed by shock and jealousy. The sight of her G-cup breasts swaying drives a deep ache into his chest, while her actions stir intense emotions within him. This unforgettable scene forever changes his feelings toward her.