After his father remarried, Hajime found himself living with his new step-mother, Saki, who was only four years older than him. He was always aroused by her beautiful, youthful body and large breasts. Studying pharmacy at university, Hajime obtained certain substances and secretly administered them to Saki as an aphrodisiac. After a few days, changes began to appear in Saki's body. Unable to resist her young step-mother's flesh, he ravaged the transformed, insatiable Saki every day.