Phineas and Ferb Season 3 Episode 18

On the fifth anniversary of getting Perry as their pet, Phineas and Ferb celebrate the day by building a Platypault and launch themselves into the air on a giant shuttlecock. They accidentally wind up at D.E. Inc., where they wreck Doofenshmirtz's Other-Dimension-ator. They then agree to help him rebuild it, not realizing they're in for a few major surprises once it's activated and they all cross into an alternate dimension. First, that Doofenshmirtz is an evil scientist. Second, his counterpart in the alternate dimension is supreme ruler of the Tri-State area. And most shocking of all, the boys' faithful pet is a secret agent! After a narrow escape from the eviler Dr. D's clutches, Phineas, Ferb, Perry, and Candace team up with their counterparts and friends in order to escape the alternate dimension and then stop Dr. D's counterpart from conquering their Tri-State area with his army of Normbots as well as Agent P's counterpart, the Platyborg.