Princess Mayuka is sent to Earth to revive the crucial Weapon when her family is overthrown in a coup d'état.
After studying the mating behaviours of Earthlings she decides to procure the help of one of her research subjects, a college student Yoshizumi.
But she accidently crash-lands on his roof then taken hostage and shot with her own gun by Yoshizumi.
After that she is brainwashed into thinking she is his younger sister.
Fortunately, Yoshizumi grows fond of Mayuka and decides to help her with her assignment to awaken the powers of Akiho the red-haired, green-eyed, appetite-driven Ultimate Weapon using somewhat primitive stimulation.