The story centers around the relationship with the protagonist's kind and beautiful stepmother, who is his father's new wife. Captivated by her bathing figure, he succumbs to temptation and sniffs her freshly removed panties, becoming aroused himself. Obsessed with her scent, he sneaks into his parents' bedroom searching for her, only to be discovered by Sumire. Rather than getting angry, she gently offers, "Shall I be your practice partner?" and begins teaching him about sex. Her warmth and tenderness intertwine, giving birth to a new, forbidden relationship.