Yoshizumi's life is turned upside down when Mayuka, an alien princess, arrives on Earth to revive an ancient superweapon to save her home planet. By using a mind-altering device to establish a sibling bond with Yoshizumi, Mayuka enlists his help to unlock the weapon—a girl named Akiho—through "primitive stimulation." After successfully activating the weapon, Yoshizumi finds himself the reluctant big brother to two otherworldly girls.
As Akiho fades away after exhausting her energy, Mayuka and Fuyuno prepare to return to their home planet following a political shift. After a passionate farewell that confirms Mayuka's genuine love for Hisashi was not the result of brainwashing, the girls depart, only to make a sudden and chaotic return that leaves Hisashi completely overwhelmed.