Two teenage siblings (Sokka and Katara) living in the South Pole discover a very strange boy (Aang), along with his pet flying bison Appa, in an iceberg. But can they trust him when he accidentally draws the war to their village's doorstep?
Katara and Sokka discover that Aang is the long-lost Avatar and must rescue him when a firebending prince named Zuko kidnaps him.
Aang is excited about going home for the first time in one hundred years, but he is shocked to find out that the temple is deserted. Aang tells Sokka and Katara about his childhood as they explore the temple, but they soon discover something that Aang never expected. Meanwhile, Zuko must make repairs to his ship and the only way he can do it is to dock his ship at his rival's naval base.
The gang arrives at Kyoshi Island and are captured by some Kyoshi warriors. When Aang reveals that he is the avatar, news quickly spreads out, eventually reaching Zuko, who then goes to Kyoshi Island to capture him.
Aang's abilities as an airbender are challenged by a strange king.
In a nearby Earth Village, the gang meets a teenage earthbender named Haru. This village lives in fear of the Fire Nation, for soldiers there capture any earthbenders they can find. After Haru is captured for earthbending, Katara is compelled to rescue him.
Aang and friends visit a village being terrorized by an angry spirit. Iroh is captured by Earth Kingdom soldiers.
To contact the spirit of the previous Avatar, Aang must reach a temple in the Fire Nation before the sun sets on the following day. There he will learn what he must do to restore balance to the world. Zuko must defy his banishment to follow Aang.
After Katara attempts to teach Aang waterbending, but realizes that he is learning it much faster than she is, she is desperate to improve her abilities. When Katara inadvertently stumbles across a waterbending scroll that can help improve her abilities at a pirate’s store, she decides to steal it, realizing that she couldn’t hope to afford it.
While traveling towards the North Pole, the group almost gets captured by the Fire Nation. However, when a charismatic young man and his band of rebels attack the Fire Nation camp and rescue them, they learn that they are trying to resist the Fire Nation’s motives to expand their territory. Katara and Aang really enjoy their time with the band of rebels, but Sokka soon discovers that they aren’t as noble as meets the eye.
While on their way to the North Pole, Aang and his friends arrive at a giant canyon where two groups of refugees are bickering over who should get to go across. Aang decides to walk across with the two groups to ensure they don’t fight one another. However, he soon learns that the two tribes have been feuding for one hundred years and they both believe their actions are justified. But when the predators inside the canyon start to come out, Aang is stuck trying to defend both tribes from each other and the vicious wildlife.
Sokka accepts a job for a fisherman who is looking for someone to help him out with his fishing. After the fisherman finds out Aang is the Avatar, Aang runs away in response to the fisherman's hostile reaction. When Katara finds Aang, he begins to reveal his life before he was frozen in the iceberg. Meanwhile, after Zuko gets into a fight with his crew, Iroh tells them about Zuko's complicated past, attempting to make both sides understand each other.
While Aang, Katara and Sokka are traveling, Sokka gets a fever. Katara and Aang try to help Sokka, when Katara also catches the fever. It's up to Aang to go up on his own to save his two friends. But before he can get the medicine he is captured by the newly promoted Admiral Zhao and put in Zhao's fortress.
When the group tries to help a man who is being attacked by a platypus-bear, they discover that the local town has a fortuneteller named Aunt Wu in it, who has been forecasting the town's fate for years. But when Sokka and Aang discover one of the fortuneteller's most vital predictions is dead wrong, they have to develop a plan to convince the stubborn townspeople that their beloved Aunt Wu was wrong.
Aang feels neglected as Katara and Sokka reunite with Bato, an old friend of their father’s from the South Pole. They begin to reminisce on their times in the past while they wait for a message to arrive from their father. Aang believes Katara and Sokka will abandon him and go out to find their father should the message arrive. However, when the messenger gives Aang the message to give to Bato, Aang betrays his friends in a way Katara or Sokka could never imagine. However, when Zuko recruits a bounty hunter named June to track Aang down, his friendship is put to the test as he must deal with the fact that he betrayed his friends while defending himself from yet another fight.
Aang and his friends go to a Fire Nation town so that Aang can witness some firebending, but their plan soon goes awry when it’s discovered that he is the Avatar. Fortunately, he's rescued by an individual who serves a firebending master named Jeong Jeong who has abandoned the Fire Nation, so Aang decides to use this time to attempt to learn firebending. The firebending master, however, seems reluctant to teach Aang.
When Aang and his friends hear rumors about surviving airbenders, they decide to stop by the Northern Air Temple on their way to the North Pole to see for themselves. What they find is a bunch of people who are not airbenders, but have mastered the art of gliding; though this upsets Aang because they have been defacing the temple. However, there was one part of the temple where nobody has been able to enter, and what Aang and his friends discover inside spells oncoming conflict.
When Aang tries to find the north pole, he is found by the north pole instead. As they are taken there, Aang and Katara are eager to find a waterbending master, only to discover that girls are not allowed to learn waterbending.
Zhao begins to launch his attack against the Northern Water Tribe. As the walls of the prestiged city begin to fall, the Avatar taps into the spirit world and seeks the help of one of the oldest spirits, in hopes of saving one of the most prominent areas of the world.
Aang returns from the spirit world to a big surprise. Meanwhile, Zhao and his forces continue to press deeper into the Northern Water Tribe. Iroh makes a powerful stand and Zuko struggles to keep his plan intact.
