Plenty of Pretty Sisters

After a coup d'état overthrows her family, Princess Mayuka is sent to Earth to recharge the Ultimate Weapon. To understand human mating behavior, she studies 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.

Now living as Yoshizumi's sister, Mayuka must help him awaken the powers of Akiho, the red-haired, green-eyed, appetite-driven Ultimate Weapon. The only way to do this is through "primitive stimulation," a process that forces them to explore intimate acts. As Yoshizumi grows fond of Mayuka, he agrees to assist her mission, leading to a strange and awkward family dynamic.