When Phineas and Ferb discover planet Pandora, they recreate the Avatar program so them and their friends can explore it. Candace follows them, and together they accidentally get dragged into the Nai'vi ranks. Dr. Doofensmirtz has stolen Nai'vi DNA, and Perry trys to stop Dr. Doofensmirtz from cloning an army of Nai'vi.
P.S. I'd like someone to post pictures of Phineas, Ferb and the gang in Avatar form, I'd do it myself, but I don't know anything about editing pictures.