In a war-torn world of elemental magic, a young boy reawakens to undertake a dangerous mystic quest to fulfill his destiny as the Avatar, and bring peace to the world.
Aang awakens to find himself aboard a captured Fire Nation ship with a full head of hair and sustaining serious injuries from the previous battle for Ba Sing Se. At his side are Katara, Sokka, and their father Hakoda who recount the events that led the four to the ship and contemplate their next move. Aang struggles to fight his own desire to face the Fire Lord before he is ready. Meanwhile, Zuko deals with his own issues as he makes his long-awaited journey home, with the fear of his father's rejection hanging over his head.
To better blend in as Fire Nation citizens, Aang enrolls in a Fire Nation school, giving himself an opportunity to learn more about the Nation and to enjoy life as a normal kid for the first time since he discovered he was the Avatar. Meanwhile Zuko, growing more and more paranoid about the Avatar's false demise, brings himself to confront the imprisoned Iroh.
When the gang arrives at a suffering fishing village, a mysterious spirit appears to help the villagers.
Feeling as though he isn't contributing enough to the group, Sokka seeks out an elusive swordsmaster from whom he can learn the ways of the blade.
Zuko, Azula, Mai and Ty Lee take a vacation to Ember Island. In the process, they come to learn more about themselves and each other as well as their desires for the future. Elsewhere, Aang and the others face off against a new adversary.
The tale of Avatar Roku and Fire Lord Sozin is revealed to Aang and Zuko by different sources. Both learn about themselves and are shown the origins of the war.
When Katara expresses her disapproval of Toph's new way to make quick cash, the group finds her too motherly and tension grows between Katara and Toph. When Katara finally decides to participate in this scam to disprove Toph's accusations, they are left vulnerable and are forced to face the dire consequences.
The gang investigates the bizarre disappearances taking place within an eerie town.
Four days prior the invasion, Aang becomes nervous over his soon-to-come confrontation with the Fire Lord. His anxiety gets the better of him as he begins to suffer from outrageous nightmares and hallucinations. Meanwhile, Zuko becomes upset to learn that he was not informed about a war meeting.
The day of the eclipse has arrived, and the invasion force gathers together to make last preparations. They infiltrate the island and the battle begins. Meanwhile, Aang searches for the Fire Lord.
As the invasion force fights their way to the Fire Nation capital, the kids help Aang find the Fire Lord in time for the eclipse. Zuko confronts his father with shocking news.
Aang and the group arrive at the Western Air Temple, with Zuko in pursuit. Zuko wishes to help the Avatar end the war by teaching him firebending.
While teaching Aang firebending, Zuko realizes that he has lost all his firebending skills. To regain his powers, Zuko and Aang must seek the original source of firebending within the ruins of a lost civilization.
Sokka wants to break his father out of jail and asks Zuko for information on Fire Nation prisons. Sokka and Zuko head for the Boiling Rock, a prison located in the middle of a volcano, to rescue Sokka's father.
Zuko and Sokka make new plans to break out of the prison with Suki, Sokka's father and some others while a shocking twist leaves Azula none too happy with her two closest friends.
Katara decides to confront the Fire Nation soldier responsible for her mother's death. Aang tries to talk her out of it, but she doesn't listen.
A play recounting the gang's adventures does not sit well with Aang and his friends, except for Toph who enjoys it very much. Meanwhile, Aang confesses his love to Katara, but the response is ambiguous.
Aang struggles to decide whether or not he has to kill the Fire Lord and, during the night, is drawn toward an island which disappears before morning. Zuko leads the rest of the group to the Earth Kingdom to meet a tracker whom they hope can find Aang. The Fire Lord crowns himself Phoenix King and Aang awakens on the island not knowing where he is.
While on the island, Aang seeks guidance from his past lives, but is not pleased with what he is told. The island, in fact, turns out to be a lion turtle from which Aang is given the guidance he had been seeking. After the tracker is unable to track Aang, the rest of the group decide they must find Iroh. Meanwhile, while tracking down Iroh, the group meets up with The Order of the White Lotus, a group that transcends the division of the four nations.
Sokka, Suki, and Toph try to stop the Fire Nation's air fleet. Meanwhile, Zuko and Azula duel in an Agni Kai and Aang confronts the Fire Lord. On the verge of losing, Aang gets hit in the back of his scar which sends him into the Avatar State.
Aang defeats the Fire Lord by bending the elements within the Fire Lord and removing his firebending abilities. After Zuko is taken down protecting Katara from Azula, Katara joins the fight and manages to restrain Azula. Afterwards, Zuko is crowned Fire Lord and promises a better future for the world, while Aang takes his place as the next Avatar. Katara finally shows her love for Aang as they kiss under the setting sun.