"Oppai Heart: Kanojo wa Kedamono Hatsujouki" follows Ryuya, a high school student whose peaceful life is turned upside down when he and his friends accidentally consume a mysterious drug called "Wild Juice," developed by their teacher. This unexpected event unleashes their wild desires, leading to intense and passionate encounters with various girls, including Ryuya's older sister, Miya. The series blends erotic excitement with a school-life setting, exploring themes of uncontrollable lust and romance.