Following a coup d'état that overthrows her family, Princess Mayuka is dispatched to Earth to recharge the Ultimate Weapon. After observing the mating habits of Earthlings, she chooses to recruit one of her research subjects, college student Yoshizumi, for assistance, 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 help her with her mission: to awaken the powers of Akiho—the red-haired, green-eyed, appetite-driven Ultimate Weapon—using "primitive stimulation".