"Rika" and "Ruka" are dizygotic twins who love their older brother and are always competing with each other. At one point, "Ruka" peeks into her brother's bath and witnesses lotion masturbation. Just here and there, she seduces her brother using swimsuit, lotion, and big boobs and exploits his brother's sperm. "Rika" who witnessed the scene begins to seduce to rob her brother.
