After her family is overthrown in a coup d'état, Princess Mayuka is dispatched to Earth to recharge the Ultimate Weapon. Following her study of Earthling mating behaviors, she chooses to recruit one of her research subjects, college student Yoshizumi, but crash-lands on his roof. Yoshizumi takes her hostage and shoots her with her own gun, brainwashing her into believing she is his younger sister. Fortunately, Yoshizumi grows fond of Mayuka and agrees to assist with her mission: awakening the powers of Akiho—the red-haired, green-eyed, appetite-driven Ultimate Weapon—through "primitive stimulation".