Princess 69: Midnight Gymnastics / New Princess 69

Dark rhythmic gymnastics is a peculiar secret competition where young creatures display their beauty and ease. The more complex and refined the technique, the stronger the athlete. Fubuki-kun, the younger brother of the main heroine Miku, becomes a witness to his sister's depraved training and gets quite aroused. Miku's classmate Tomomi-chan notices this and starts using our hero as a sex doll, and then offers to become his "trainer" in dark gymnastics.