The boys want to invent something for Candace, and decide to take her to a universe where her favorite cartoon character, Ducky Momo, exists.
Things complicate when their invention malfunctions, and instead they end up opening a gateway in the middle of Ponyville, allowing both humans and ponies to cross over between the two universes.

Will Buford express his deep love for ponies?
Will Phineas and Ferb get busted?
Will Doofenshmirtz manage to conquer the Tri-state area (and Equestria)?

The answer to all these is probably 'no', but it is still sure to be interesting nonetheless!