A cute princess Mayuka is sent to Earth to recharge the Ultimate Weapon after her family is overthrown by coup. She crash landed on the roof of a home of college student Yoshizumi. Taken hostage and shot with her own gun by Yoshizumi, she is brainwashed into thinking she is his younger sister. Fortunately, Yoshizumi grows fond of Mayuka and decides to help her with her mission: to awaken the powers of Akiho-the red-haired, green-eyed, appetite-driven Ultimate Weapon by the power of sex with creampie.