Across the 2nd dimension

Phineas and Ferb celebrate the fifth anniversary of their pet platypus, Perry, with a platypult, but their creation causes them to end up in Dr. Doofenshmirtz's lair where they unknowingly help him launch the Other- Dimensionator, which zaps Phineas and Ferb into a new world where the face their alternate selves and even an alternate Dr. Doofenshmirtz.