<P> 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 . </P> <P> 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 . </P> <P> 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 . </P> <P> 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 . </P>

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