When Mom is out of Doonkleberries for her famous Doonkleberry pie, the boys set out to find some for her. Meanwhile, Perry and Dr. Doofenshmirtz find themselves across the ocean as Doofenshmirtz returns home to renew his Druesselsteinian driver’s license. Back in Danville, Candace is convinced she has discovered the answer to her busting problem.

Brigitte, Josette, and Colette, the visiting Fireside Girls from France, help Isabella capture the American Grizzly Bear, one of the world's most dangerous creatures. Buford's plan to jokingly dress up as the bear backfires when he discovers the identity of Brigitte, throwing him off his game. Back at their grandparents' cabin, Phineas and Ferb create the most amazing "run-through-the-sprinkler" experience. Meanwhile, Dr. Doofenshmirtz's loathing of pretzels drives him to create the "De-Twist-Inator" to untwist every pretzel in Danville.

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.

Phineas and Ferb create their own flying saucer to help solve the mystery of a recent rash of crop circles. Meanwhile, Candace tries to give up her tattling habit by taking a "me" day, and across town, Dr. Doofenshmirtz challenges the local pizzeria's "37 Minutes or it's Free" rule by using his "dough-blow-inator" to generate a powerful current of air to hold the pizza dough aloft indefinitely.

The guys help their dad unearth a missing piece of Danville history; Doofenshmirtz tries to make himself younger to win a children's film festival.

Candace finds it difficult to bust Phineas and Ferb when her brothers build nanobots that can assemble into any object they can imagine. Meanwhile, Dr. Doofenshmirtz has jury duty, so Norm takes over the day’s evil scheme -- to destroy Danville by using weapons.

Agent P's fate is finally revealed in the action-packed finale of the two-part television special that left Phineas, Ferb and the gang hanging...literally off an unchartered gorge and Candace frantic over a phone call with Jeremy that left their relationship status in limbo. Now, with Agent P nowhere in sight, Dr. Doofenshmirtz and Major Monogram square off against evil Carl who controls a set of familiar-looking robots to take over the OWCA, in one of the biggest battles yet.

When one of Dr. Doofenshmirtz's "-inators" zaps him and Agent P into the middle of nowhere, they must embark on a comical journey filled with twists and turns to return home.

Agent P quickly figures out there is more to Norm's odd reality show that celebrates the eventful life of his creator Dr. Doofenshmirtz, which is recreated through flashbacks and special guests.

Candace is put in charge of Phineas and Ferb just before they seize the opportunity to make the most of Danville's electricity blackout while Dr. Doofenshmirtz tries to use pity to his advantage.