When Baljeet becomes concerned that he is losing his mental edge, the boys invent a cerebral stimulatory hat, or "Brain booster," to help him. But when the hat exceeds expectations and makes Baljeet exponentially smarter, he suddenly views everyone else as beneath him and becomes a threat to civilization by taking Earth's atmosphere and bringing it to the moon. Meanwhile, Dr. Doofenshmirtz plans to use his "Key-Find-Inator" to collect all the keys in Danville in hopes that it will lead to the Key to the City.

With Fall around the corner, Phineas and Ferb decide to invent "S'Fall," a colorful world full of giant leaf piles and gourds. Meanwhile, over at Evil Inc., Dr. Doofenshmirtz is mentoring Rodrigo, a budding student of evil. Unfortunately, Rodrigo truly is evil, and when he captures Dr. Doofenshmirtz and Agent P., it's up to Major Monogram's son, Monty, to save the day.