While all of their friends are at Disney World, Phineas and Ferb build a portal to another TV show. Intending to go to the show Chowder, the boys miscalculate something and wind up in the show Avatar: The Last Airbender. Once there they stumble accross Toph who has just finished having tea with Iroh. They talk to her some, and are on their way, or so Toph thinks. The boys secretly follow her. Once the gang fights Azula, they realize they left the portal open and Azula steps though. But Phineas and Ferb force her back in once they realize she can't firebend because bending doesn't exist in their show. Meanwhile, Doofenshirtiz has also gone to that world to Platypus-bearnapp all the platypus-bears. So Perry and Bosco team up and beat up Doofenshiritiz up to the point where he loses his memory. After that The boys say their goodbyes and go back to their show.
The "Too Young" Line
Phineas: Aren't you a little young to be the Avatar?
Aang: Yes, yes I am.
Phineas: Yeah, I get that a lot.
Phineas: I wish we had one of those Platypus-bears.
Ferb: Platypus-bears are the only mammals in the Avatar universe to lay eggs!
Said by Toph when she meets Phineas and Ferb.
Isabella: Hi guys, watcha doin?
Phineas: joining Avatar Aang and his allies. You wanna tag in?
Perry's entrence to his lair
Goes throgh a tube at lake Nose then goes though another tube and ends up in lake Lagoi.
" Doofenshirtiz is in Avatar!"