Aang and his friends arrive at the Earth Kingdom city Omashu. Sokka and Katara look on in amazement and can't believe they have buildings that don't melt from the heat. Aang hurries to the city, but Katara thinks he needs to hide his identity as the Avatar for his safety. He uses some of Appa's shed fur as a wig and mustache and adopts a fake, old man voice, crouching and using his staff as a walking stick.
As they approach the city's walls, Aang remarks Omashu is one of the friendliest places in the world. But guards at the city wall are giving a Cabbage Merchant trouble about trying to bring rotten cabbages into the city, and Earthbend his vegetable-filled cart into the valley below, causing the merchant to scream "No! My cabbages!" in dismay. Aang and the others approach, and the guards demands to know who they are. Aang invents the name Bonzu Pipinpadaloxicopolis The Third for himself and says Sokka and Katara are his grandchildren, and the guards let them into the city. Aang shows them the Omashu delivery system, which consists of miles of tubes and chutes which send packages throughout the city via Earthbending. He also tells them one hundred years ago while visiting, Aang and his eccentric childhood friend Bumi would slide down the Omashu delivery system for fun. Bumi told Aang to always open his brain to the possibilities in life, and Aang affectionately called Bumi a "mad genius".
Aang promises Katara they will go to the North Pole once they have a ride down the chutes. They go to the highest point in the city and slide down, almost getting killed by a package of spears following behind them as they slide. Aang derails them from the chute and their bin scrapes across the rooftops of several houses. Panicking, they can't stop and crash into the Cabbage Merchant's cart, who screams once again in dismay for his ruined cabbages. They are surrounded by guards and taken captive to be punished by the King of Omashu. An aged old man, the King looks for a while at Aang, then decides instead of punishment that they shall be thrown a feast, to the surprise of the guards. At the feast, the King inquires about where Aang came from, which Aang replies (with a lie) that he came from Kangaroo Island. The King cracks a joke, saying he's heard that place is "really hoppin'," which only Sokka laughs at. Feigning his departure from the table, the King suddenly throws a chicken drumstick at Aang, causing him to catch it with Airbending out of reflex, revealing to everyone that he is the Avatar.
Aang and the others try to leave, but are detained by the guards. The King says the Avatar shall face three deadly challenges tomorrow to win the freedom of him and his friends, and locks them in a chamber.
The next day, Aang wakes and finds Sokka and Katara are missing. The King holds them hostage and promises to return them once the challenges are completed. Aang threatens the King and demands he return his friends immediately, but the King brings forth Sokka and Katara and has rings of Jennomite placed on their fingers. Known as "creeping crystal," the crystals will grow and grow until they cover their entire body, and only the King can undo it. Aang submits to go through with the King's challenges. The first challenge, Aang must simply retrieve a key hanging from a chain within a powerful, cascading waterfall. But it isn't as simple as it seems. The force of the water is too powerful for Aang to simply dive in or approach from below. Even using Airbending Aang can't get to the key. The crystal is now covering Katara and Sokka's arms. Aang resorts to breaking the tip of a stalagmite off, and using Airbending shoots it like a dart into the waterfall, breaking the chain and carrying the key to the King, who was watching from a balcony.
For the second challenge, Aang must return the King's lost pet Flopsie to him. Outside, Aang sees a bunny and chases after it, but a large monster chases after Aang. The bunny escapes and the monster closes in, but Aang has a sudden idea. He turns to face the monster, and realizes the monster is actually Flopsie. Flopsie picks Aang up tenderly and gives him a huge lick. He drops Aang and runs to the King, who rubs the monster's belly. Katara and Sokka are covered from head to ankle in crystal, and Aang demands the final challenge begin.
In a large arena, Aang must battle an opponent, but he is allowed to choose which opponent to fight, but the King warns him to choose carefully. He can pick one of two nasty-looking gladiator-like warriors, or the old, frail King. Aang chooses the King, who declares he chose wrong, removing his robes to reveal a surprisingly muscular physique for such an old man. Aang recoils, realizing his mistake. The King claims he is the most powerful Earthbender Aang will ever meet, and begins the battle. The King immediately launches several boulders at him which Aang dodges, but the King taunts him for such a predictable Airbender tactic. As they fight, the King continues to taunt Aang for his unwillingness to fight back. Eventually Aang begins to counter the King. With tremendous effort, the King tears a massive chunk of earth out of the ground and holds it over his head. Aang screams in terror and begins to run in circles, Airbending a tornado. The King throws the massive mound, but the tornado redirects it back at him. He bisects it neatly in two. Aang appears between the rock pieces and pins the King with his staff.
The King concedes defeat, but before releasing his friends, requires the Avatar to answer one last question: What is the King's name? Recalling the point of all the challenges, Aang realizes they all forced him to think differently then he usually would. He realizes who the King is, and calls his old childhood friend Bumi, now the old King of Omashu, a mad genius. The two embrace, and Bumi declares Aang hasn't changed a bit, literally. Bumi shatters the crystal encasing Sokka and Katara, and eats a piece, revealing Jennomite is made from rock candy.
Bumi wishes Aang luck in his quest to defeat the Fire Lord, and hopes when he confronts him that he'll think like a mad genius. Aang tells Bumi he now has a challenge for him, and the two of them go sliding down the Omashu delivery system like the old days, which results in them crashing into the cabbage merchant's cart, just like before.
