Tsuyoshi, preparing for university entrance exams in Tokyo, stays at his brother's house. He reunites with his sister-in-law, Chihaya Maria, a fitness instructor with a slender figure and incredibly attractive G-cup breasts. Aroused by her, Tsuyoshi decides to buy aphrodisiacs online and prepares everything. While exercising in the living room, Chihaya Maria accidentally drinks the aphrodisiac. Due to its effects, she begins to feel hot and unconsciously touches her genitals and masturbates. Seeing this, Tsuyoshi assaults her, and she actively begs for his penis…