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troperville

As the gang continues on their journey to the North Pole, they stop at the city Omashu, a place Aang remembers from his child-hood for his friend Bumi and its elaborate earth-bending controlled mail system. The group takes advantage of said mail system for fun, but their antics create massive property damage, leading to them being captured by the Royal Guard. The slightly insane King of Omashu realises that Aang is the Avatar, and imprisons Katara and Sokka in living crystal; if Aang wants to free them, he shall have to complete three tasks.

The first test is to fetch a key (For the king's lunchbox, apparently). Problem is, the key is kept in the center of a downpour of water above a pit of stalagmites. The pressure too great to just climb up the ladder, Aang is able to get the key by flinging a stalactite at it, knocking it out of the water and over the king's head.

The second test is to fetch the king's pet, Flopsy. Aang is led to a pit where a small bunny is kept...and a giant gorilla-goat! After some comic chasing, Aang realizes that the goat-rilla is Flopsy, who acts very docile when called for.

The third test is a duel. Two fierce-looking warriors come out, and the king tells Aang to choose his opponent. Trying to be clever, Aang tries to Take a Third Option and challenge...the king! Too bad the king has some serious muscle underneath all those robes and wrinkles!

King: You thought I was a frail old man, but I'm the most powerful earthbender you'll ever see!

The duel starts off badly for Aang, who continually has to avoid the king's mighty earthbending. The king for his part mocks Aang use of traditional airbender tactics of evasion, goading the Avatar to be more creative. Aang eventually tries to use his airbending to redirect the king's earthbended boulders, becoming more offensive culminating in a self-made tornado. The king soon calls the duel off and congratulates Aang for his fighting spirit, but tells that the Avatar has one more challenge (After all, what good are tests if you don't learn anything?); What is the king's name?

Aang confers with his Water Tribe comrades, and after dismissing Sokka's suggestion wonders about the previous tests. To pass all of them, Aang had to open his mind to all of the possibilties...Aang realizes what the king's name is. Confronting the leader of Omashu, Aang goes up to him and...gives him a big hug.

Aang: You're a mad genius, Bumi.

The two friends reunited, Bumi removes Katara and Sokka's crystal capsules. He also reveals why he tested Aang; First off, it's fun. Second, in order to defeat Fire Lord Ozai and restore balance to the world, Aang will also have to think like a mad genius. The episode ends with Aang and Bumi playing with the city's mail system, at the expense of a certain merchant of cabbage...

Tropes

Air Vent Passageway: Zig Zagged. Aang suggests escaping through the air vents, but Sokka notes that they couldn't fit. Aang says that even though they can't fit, Momo can. However, Momo just gets stuck halfway through the air vent hole.
Arson, Murder, and Jaywalking: The Gaang are hauled up in front of the King for "vandalism, traveling under false pretenses and malicious destruction of cabbages."
Chirping Crickets: Some guy coughing.
Crouching Moron, Hidden Badass: Bumi
Dishing Out Dirt: Hey, King Bumi's an Earthbender, its his thing.
Fluffy Tamer: King Bumi.
Fluffy the Terrible: "Flopsy" turns out to be the hulking gorilla-goat, not the little cute pet. Though it winds up being just as docile as you'd expect something named "Flopsy" to be.
Fruit Cart: The Cabbage Merchant and his cabbages.
Late Arrival Spoiler: The King being Aang's old friend Bumi is the big Reveal of the episode, but as you can tell we're not even bothering spoilering it.
No, Mr. Bond, I Expect You to Dine: When the protagonists are arrested, the king proclaims their sentence: "THROW THEM... a feast!"
Off with His Head!: The cabbage merchent suggests this punishment.
Cabbage Merchant: Off with their heads! One for each head of cabbage!
Overly-Long Name: Aang initially identifies himself as "Bonzu Pipinpadaloxicopolis III".
Pun: Quite a few, thanks to King Bumi.
Recurring Extra: The poor cabbage merchant's first appearance.
Secret Test of Character: The King was Bumi, and the tasks were partly a test to see if Aang could think creatively.
Take a Third Option: When presented with a choice of warriors to fight, Aang chooses Bumi instead. Subverted when Bumi turns out to be stronger then the scary-looking guys, and he all but calls out Aang for taking the predictable out.
Unpronounceable Alias: See Overly-Long Name above. Though Katara and Bumi can keep up with it just fine.