Aang leads Katara and Sokka to the major Earth Kingdom city, Omashu. He is excited to visit the city again, as he used to frequently do more than one hundred years ago. Meanwhile, the Water Tribe siblings are in awe of the city's immensity. Aang starts rushing towards the city, but Katara and Sokka stop him, and suggest he disguise himself to avoid trouble. Aang complies, crafting a wig and false mustache from Appa's fur and adopting the comical persona of a stereotypical elderly man, while Momo hides in his wig. Despite Aang's assurance that the people of Omashu are very friendly, he and his friends soon discover evidence to the contrary. The gate is guarded by three confrontational Earthbenders, who destroy the cart of an unfortunate cabbage merchant and then threaten the disguised Aang. The Avatar's quick thinking and aggressive response to the threat, however, quickly secures their safe passage into the city.
Once inside the city, Aang and his friends are greeted by magnificent sight of Omashu's delivery system- an immense network of chutes and tubes that carry carts filled with various items throughout the city and powered by earthbending and gravity. Aang recalls one of his friends from his childhood, a slightly crazed Earth Kingdom boy named Bumi, who found a second use for the chutes- using them as a gigantic super slide. Aang convinces Katara and Sokka to ride down the delivery system with him before they continue on their journey. Although the siblings agree, they soon regret their decision, yelling in terror while Aang whoops with glee as they speed down the slide. The ride soon becomes dangerous when another cart bearing a set of spears comes up behind the group and very nearly injures Sokka and Katara, and Aang is forced to divert his cart's course. The cart runs along several rooftops, through various buildings, and into and out of other chutes, causing more and more damage along the way until crashing into a cart filled with cabbages- belonging to the same unfortunate merchant from the city's gate.
Several guards bring Aang, Katara, Sokka, and Momo before Omashu's king, an elderly and frail-appearing man. The cabbage merchant angrily demands that the young ones be beheaded, but the head of the guards silences him, and asks for the king's judgment. The king studies the three youngsters briefly, and deems that a feast is in order, to the surprise of all those present. However, after some time, the king's true reason for throwing the feast are revealed, as he tricks Aang into revealing his airbending abilities, thus blowing his cover. The king states that the following day, Aang must face three challenges. Until that time, he orders Aang and his friends to be kept in the prison chambers. Despite the odd comfort that their quarters provide, Aang, Katara, and Sokka attempt to escape, though to no avail; they shortly fall asleep.
Aang is later awakened by the entrance of one of the guards, and finds that Katara and Sokka are gone. The guard commandeers Aang's staff, then escorts him before the king. Aang angrily demands to know where his friends are, and the king complies- Katara and Sokka are revealed, each detained by a guard, who slip rings of a strange crystal onto their fingers. The king explains that the rings are made of Jennamite, also known as "creeping crystal", a mineral that grows at an incredible rate. Katara and Sokka, unable to forcefully remove the rings, will inevitably be completely covered by the crystal unless the king decides to stop the process, which he will only do if Aang successfully completes the three deadly tasks assigned to him. Aang begrudgingly submits.
For his first challenge, Aang is taken to an underground cavern filled with stalactites and stalagmites and featuring a waterfall. In the middle of the waterfall, the king's "lunch-box key" is dangling from a chain over a larger stalagmite. With the Jennamite already covering Katara and Sokka's forearms, Aang must retrieve the key. He nimbly jumps from stalagmite to stalagmite, and quickly makes his way towards the waterfall. He dives head-on into the crushing column of water, and climbs to a ladder secured to the large stalagmite which leads directly up to the key. However, the tremendous pressure of the water soon causes him to lose his grip, and he only barely manages to prevent himself from being skewered by one of the smaller stalagmites. Aang tries a slightly different approach by clinging to the stalactites above and leaping at the key from a higher angle, but the waterfall prevents him from reaching the key in this way as well. After this failed attempt, Aang has a brief epiphany, and snaps off the top of one of the stalagmites. He then heaves the rock like a spear at the waterfall, and Airbends an arc of wind after it. The wind momentarily beats a path through the waterfall, and the stalagmite snags the chain holding the key, depositing it just above the king's head. However, despite Aang's reinforced demand, the king still has two more challenges for the Avatar. The king states that he has lost his dear pet, Flopsy, and orders Aang to recover him. Aang momentarily leaps into a small valley, where a rabbit-like creature with enormous, floppy ears is perched on a rock. Before Aang can recover the animal, however, a large, white-furred beast with the crossed features of a gorilla, a rabbit, and a goat appears, frightening away the smaller animal. Aang runs to catch the rabbit-like animal, but it soon dives into a burrow, and he is unable to reach it. Aang quickly realizes that this means that the animal is not the Flopsy that he was sent to find. He hesitantly turns to the beast still chasing him, and addresses it as Flopsy. This reduces the previously ferocious creature to happily clutching and licking Aang, before it bounds off to receive a belly rub from the king. As the king and Flopsy happily play for a moment, Aang checks on Katara and Sokka, who are now almost entirely covered in crystal.
The king leads Aang and his friends to a balcony overlooking a vast arena almost entirely constructed of rock and stone. He states that Aang's final challenge is to win a duel, for which he will be allowed to freely choose his opponent. However, after a pair of fierce-looking warriors come forth; one a slim muscular man with a crescent sword and a nasty smile, the other a mammoth man with a large axe, Aang instead chooses to battle the king himself. The king merely smirks, appearing to have expected this. He then straightens his hunched figure, and casts off his dressy robes, revealing a muscular form. He then stomps the ground, sending a surprised Aang flying into the arena with an earthbending move, before leaping in to join him. The king states that despite his appearance, he is actually the most powerful Earthbender in the world. Just before the battle commences, he allows Aang to reclaim his staff to provide aid he will surely need. The king controls the majority of the battle from the beginning, launching increasingly powerful barrages of earth at Aang while erecting various obstacles and barriers to keep the Airbender at a distance and render his return attacks useless. All the while, the king taunts Aang, causing him to step up his efforts again and again. In spite of this, the king is rarely hit by the Avatar's attacks, while Aang is constantly forced on the defensive. Despite his fantastic evasive abilities, however, Aang is struck several times and knocked to the ground. Switching over to the offensive, Aang makes a daring charge at the king, who nullifies it with a high-level earthbending technique that turns the very ground beneath him into sand. Avoiding being crushed between two slabs of rock, Aang is able to escape from the pit and finally land a blow on the king. Eventually, with tremendous effort, the king tears away a tremendous mass of rock, including the balcony on which he and Aang began their fight. In response to the upcoming attack, Aang begins to run rapidly in circles, creating a cyclone with in conjunction with Airbending. When the king finally launches the attack, Aang's cyclone redirects the immense boulder right back to the king, and he is forced to bisect the mass of rock to defend himself. Aang capitalizes on this distraction by closing their distance and pointing his staff to the king's chest, signifying the ending of the duel. However, the battle is not a complete strategic victory for Aang as the king smirks and redirects the Avatar's attention to directly above them, where a large chunk of rock is hovering over their heads. However, despite the effective stalemate, the king congratulates Aang, and tosses away the rock. Aang and the king rejoin Katara and Sokka, and the king states that the Avatar has passed all of his tests. However, he is not ready to free Aang's friends just yet. Rather, he instructs Aang to answer one question, before he sets Katara and Sokka free- Aang must correctly guess what the king's name is, in the few minutes that remain before Katara and Sokka are totally encased in Jennamite. Upon Katara's advice to think of the three challenges and what he learned from them, Aang reflects that he was forced to think differently than he normally would, and soon comes to the conclusion he is seeking. When he confronts the king, he correctly identifies him as Bumi, his friend of a hundred years ago. Aang and Bumi happily reunite with a hug, before Katara and Sokka arrive and stressfully ask for assistance- their faces are almost covered in Jennamite. Bumi shatters the crystals surrounding them with Earthbending, revealing that Jennamite is really just rock candy, commenting it as delicious while Momo also tastes the Jennamite, as well as the reason he challenged Aang- to prepare the young Avatar for his journey to master the four elements and defeat Fire Lord Ozai. Before Aang leaves, he and Bumi ride Omashu's delivery system together, as they did a century before. And they destroyed the Cabbage Merchant's stand again, to his misfortune.