A mysterious new boy is seen around town, and Julia soon discovers him to be an alien, and the son of one of her father's greatest old enemies: Zim of the Irken Empire! Now, she, along with Lizzie and Thomas, must somehow convince the rest of their family that Zor is an alien and then stop him before he brings about the destruction of mankind. Meanwhile, Phineas and Biz have their hands full trying to repair the Flynn and Co. hotel on the moon seen in The First Family in Space.