Doofus Khan tries to take over the Tri-Province Area by abducting Princess Isabella.
Department of Redundancy Department: Unclimbable Mountain of Unclimbableness and the Uncrossable River of Uncrossableness, both of which were named by the Redundant Scribe of Redundantness. Candace calls the mountain the "Stupid Mountain of Stupidness" and the scribe says that's how he originally intended to call it. The scribe is redundant whenever he greets someone or apologizes to someone.
Duck!: When Regent Monogram tells General Carl to duck, Carl assumes Monogram mistook Doofus Khan's wind-up dragon for a duck and ends up getting hit by the dragon's tail.
Noodle Implements: It's not shown how Phineas and Ferb get past Doofus Khan's traps, but a comment from Phineas suggests that a yak bladder was needed at some point.
Suspiciously Specific Denial: When Regent Monogram asks about Princess Isabella's whereabouts, Doofus Khan explains that she certainly didn't escape and thus force him to alter his plans.
Tempting Fate: When Doofus Khan neutralizes the kids' wind-up terracota warrior by removing its key, they try to do the same to his wind-up dragon but can't find the dragon's key. After Doofus Khan says that only a great master and warrior can know where he has hidden his key, Master Perry the Platypus shows up.
Time Skip: After Doofus Khan is defeated, Phineas and Ferb decide to build a Great Wall to prevent future invaders from entering the Tri-Province Area. The next scene is from 40 years later, when they finish building it.