Avatar: The Last Airbender is set in a world—adjacent to a parallel spirit world—that is home to humans and hybrid animals. Human civilization is divided into four nations, named after the four classical elements: the Water Tribes, the Earth Kingdom, the Fire Nation, and the Air Nomads. Each nation has a distinct society in which select people, known as “benders” (waterbenders, earthbenders, firebenders and airbenders), have the ability to telekinetically manipulate and control their nation’s element using martial arts.
Only the Avatar has the ability to bend all four elements. The Avatar, who may be male or female, is an international arbiter whose duty is to maintain harmony among the four nations, and act as a mediator between humans and spirits. When the Avatar dies, their spirit is reincarnated into the next of the four nations in what is known as the Avatar cycle: the Fire Nation, Air Nomads, Water Tribe and Earth Kingdom. An Avatar is required to master each bending art, beginning with the element of their home nation, and proceeding to learn the others in the order of the Avatar cycle, starting at the age of sixteen. Avatars also possess the ability to enter a condition known as the Avatar State, in which they gain the knowledge and abilities of all past Avatars. Although it is when they are most powerful, if an Avatar were ever to be killed while in the Avatar State, the reincarnation cycle would end and the avatar would never again be reborn.
More than a century before the events of the series begin, Fire Lord Sozin, ruler of the Fire Nation, planned a world war to expand his nation’s territory and influence. He was prevented from carrying out his plans by Avatar Roku, who was born in the Fire Nation. After Roku’s death, the Avatar was reincarnated as an Airbender named Aang. Because the Avatar was now a child, Sozin saw his chance and proceeded with his militant plans. At the age of twelve, Aang learned about his avatar status because of the threat of Sozin’s war. Afraid of his new responsibilities and of separation from his mentor Gyatso, he fled his home on his flying bison, Appa. Aang was forced into the ocean by a storm; he entered the Avatar State and encased himself and Appa in suspended animation in an iceberg near the South Pole for one hundred years. Sozin, knowing the avatar’s reincarnation cycle mandated an Air Nomad was the new Avatar, carried out a genocide against the Air Nomads during the passage of a once-a-century comet that increased the Firebenders’ power, and continued his world conquest.
In the present day, as the Fire Nation continues its war against the other nations, Katara, a fourteen-year-old waterbender of the Southern Water Tribe, and her older brother, Sokka, find and revive Aang and Appa. Aang learns about the war, and the siblings join him to reach the Northern Water Tribe at the North Pole so he and Katara can learn waterbending. Aang’s return attracts the attention of Prince Zuko, the exiled son of the current Fire Lord Ozai, who pursues them, accompanied by his uncle Iroh. Aang is also pursued by Zhao, a Fire Nation admiral who intends to win Ozai’s favor and rob Zuko of his redemption. En route to the North Pole, Aang learns about the genocide against his people when he visits the ruins of the Southern Air Temple. During the winter solstice, Aang meets the spirit of his predecessor, Avatar Roku, and comes to terms with his responsibilities. With the Northern Water Tribe, Aang and Katara learn advanced waterbending from Master Pakku; Sokka falls in love with the chief’s daughter, Princess Yue. Zhao’s navy lays siege to the tribe, seizing the mortal forms of the ocean and moon spirits—the source of waterbending—and causing a lunar eclipse. Zhao kills the moon spirit to deprive the waterbenders of their abilities but Aang joins with the ocean spirit to drive off the enemy fleet while Yue sacrifices her life to revive the moon spirit. When Ozai hears about his older brother Iroh’s resistance to Zhao, he sends his daughter Azula to capture Iroh and Zuko.
After leaving the Northern Water Tribe, Katara continues teaching Aang waterbending while the group searches for an earthbending teacher. They meet Toph Beifong, a twelve-year-old, blind earthbending prodigy who wants independence from her upper-class family. Pursued by Princess Azula, Zuko and Iroh lead new lives in the Earth Kingdom as wanderers and refugees, settling in the capital city Ba Sing Se. At a library guarded by the spirit Wan Shi Tong, Aang and his group learn an imminent solar eclipse could let them stop the Fire Nation before Sozin’s Comet arrives. They journey to Ba Sing Se to inform the Earth King of this information. In the city, they find that the Earth King Kuei is a puppet ruler, manipulated by Long Feng, leader of the Dai Li secret police. After Aang’s group exposes Long Feng’s political machinations, Toph is captured but escapes by learning to bend metal. The Dai Li join Azula to instigate a coup d’état of Ba Sing Se, and Zuko, who has spent his time in Ba Sing Se trying to come to terms with his identity, sides with his sister Azula. During a face off in the catacombs underneath the ancient city, Azula nearly kills Aang, forcing the protagonists to retreat with help from Iroh and leaving the Earth Kingdom under the control of the Fire Nation.
Aang emerges from a coma to find his friends and allies disguised as soldiers on a Fire Nation ship, preparing to invade the Fire Nation capital during the solar eclipse. The invasion is at first successful but Aang and his friends cannot find Ozai and are forced to retreat. Zuko learns about his father’s intention to destroy the Earth Kingdom during the return of Sozin’s Comet; he begins to regret his decision, and abandons the Fire Nation to join Aang and teach him firebending. As the comet approaches, Aang, a pacifist, wrestles with the possibility that he will have to kill Ozai to end the war. As he, in seclusion, consults the spirits of his predecessors for advice, Katara and the others encounter Iroh, who is leading a secret society called the Order of the White Lotus. The Order liberates Ba Sing Se. Sokka, Toph, and the warrior Suki attack the Fire Nation’s airships, while Zuko and Katara confront Azula to prevent her from being crowned as the new Fire Lord. As the comet arrives, Aang confronts Ozai but cannot get the upper hand until Ozai triggers Aang’s connection to the Avatar State. Aang strips Ozai of his firebending powers. Zuko is crowned the new Fire Lord and arranges an armistice, establishing peace in the world.