Imouto de Ikou!

After a coup overthrows her family, Princess Mayuka is dispatched to Earth to reactivate the Ultimate Weapon. Her research into Earthling mating habits leads her to select college student Yoshizumi as an assistant, but her landing on his roof goes awry. Yoshizumi takes her hostage, uses her own weapon against her, and implants a false memory that she is his younger sister. Developing an attachment to Mayuka, Yoshizumi agrees to aid her objective. The mission involves rousing the powers of Akiho, the red-haired, green-eyed Ultimate Weapon known for her appetite, through the application of primitive stimulation.