Avatar: The Last Airbender takes place in a world that is home to humans, fantastic animals, and spirits. Human civilization is divided into four nations: the Water Tribe, the Earth Kingdom, the Fire Nation, and the Air Nomads. Each nation has its own natural element, on which it bases its society. Furthermore, people known as Benders have the power and ability to control and manipulate the eponymous element of their nation using the physical motions of martial arts. The show's creators based each Bending style on a style of real-world martial art, leading to visual differences in the techniques used by Waterbenders (Tai chi), Earthbenders (Hung Ga kung fu, for the most part), Firebenders (Northern Shaolin kung fu) and Airbenders (Baguazhang).
At any given time, there is only one person alive in the world of Avatar who is capable of bending all four elements: the show's eponymous Avatar, the spiritual entity of the planet manifested in human form. When an Avatar dies, he or she is reincarnated into the next nation in the Avatar Cycle, in the order of the seasons. Legend holds the Avatar must master each bending art in seasonal order as well, starting with their native element. For the Avatar, learning to bend their opposite element can be extremely difficult; the example shown in the series is Aang's inability to stand his ground head-on while Earthbending, his Airbender training having placed emphasis on circling, approaching from new angles and adapting on the fly.
The Avatar possesses a unique power and ability called the Avatar State, which endows the Avatar with all the knowledge and abilities of all past Avatars and acts as a self-triggering defense mechanism, although it can be made subject to the will of the user through various methods, such as extensive trial and training. If an Avatar is killed in the Avatar State, the reincarnation cycle will be broken, and the Avatar will cease to exist. Through the ages, countless incarnations of Avatar have served to keep the four nations in harmony, and maintain world order. The Avatar serves as the bridge between the physical world and the Spirit World, allowing him or her to solve problems that normal benders cannot.
Season One (Book One: Water)
One hundred years before the start of the series, a twelve-year-old Airbender named Aang learns he is the new Avatar. Fearful of the heavy responsibilities of the position, and being separated from his beloved mentor, Monk Gyatso, Aang flees from home on his animal guide, a giant six-legged flying bison named Appa. During their flight, they become caught in a fierce storm and crash into the ocean; Aang's protective Avatar State is triggered, and it encases the pair in an iceberg, in suspended animation. Meanwhile, Fire Lord Sozin, who killed the previous Avatar because he would have acted to stop his war plans, begins a genocidal assault on the Air Nomads. Sozin knew that the new Avatar would be an Air Nomad, and eliminated them in order to ensure his domination of the world would not be interrupted. Ironically, Aang is the only Air Nomad to survive the attack.
One hundred years later Katara, a fourteen-year-old Waterbender girl, and her older warrior brother, fifteen-year-old Sokka, free Aang and Appa from the iceberg. The three travel to the Northern Water Tribe so Aang and Katara can learn Waterbending. While on their journey, Aang and friends visit the Southern Air Temple where Aang discovers that the Fire Nation wiped out the Air Nomads. At the Southern Air Temple Aang meets his Avatar guide, Avatar Roku. On their journey, the trio is constantly pursued by Prince Zuko, the exiled son of Fire Lord Ozai, who can only reclaim his honor and throne if he captures the Avatar. Zuko travels with his uncle Iroh, a legendary Fire Nation general and the older brother of Ozai. Competing with Zuko for the Avatar is Admiral Zhao, who leads a Fire Nation assault on the Northern Water Tribe. The attack is repelled thanks to Aang and his friends, but after the siege is ended, the Fire Lord tasks his daughter Azula to capture Zuko and Iroh, who are now considered traitors to the Fire Nation.
Season Two (Book Two: Earth)
After leaving the North Pole, Aang masters Waterbending under Katara's tutelage. Searching for a new Earthbending teacher, the group meets Toph, a blind Earthbending prodigy who teaches Aang how to "see" using Earthbending and vibrations. Meanwhile, Zuko and Iroh, now fugitives from the Fire Nation, attempt to lead new lives in the Earth Kingdom. Zuko, with the help of his uncle, tries to come to terms with his troubled past and his obsession with capturing the Avatar. Aang and his friends discover that an upcoming solar eclipse will deprive Firebenders of their bending, leaving them open to invasion and giving Aang his chance to defeat the Fire Lord, but in learning this Aang's Sky Bison is lost to a group of Sandbenders.
Azula and her two friends Mai and Ty Lee pursue Team Avatar, who struggle to reach Ba Sing Se, the Earth Kingdom capital, and tell the Earth King of the eclipse. After capturing and impersonating the Kyoshi Warriors, friends of Team Avatar, Azula persuades an elite group of Earthbenders called the Dai Li to instigate a coup d'état, allowing the Fire Nation to capture Ba Sing Se. In a final confrontation, Zuko sides with his sister, who promises to restore his honor. Azula mortally wounds Aang, who is in the Avatar State, with a lightning blast. Iroh helps Aang and Katara to escape after Zuko betrays him, and is imprisoned by the Dai Li for his actions. Katara revives Aang with special spirit water from the North Pole, but his seventh chakra is blocked and he cannot enter the Avatar State.
Season Three (Book Three: Fire)
Aang awakens to find the group disguised as Fire Nation soldiers heading West on a Fire Nation ship, while Zuko has been restored to his position as crown prince and Iroh is imprisoned as a traitor. Sokka has planned a small-scale invasion of the Fire Nation to capture the Fire Lord's palace and defeat Fire Lord Ozai, taking advantage of the solar eclipse. The invasion will be staged by a ragtag group of benders and warriors whom Aang has helped along his journey. Initially the invasion proceeds as planned, but Aang fails to find the Fire Lord before the eclipse ends. The invasion ultimately fails, and only Aang and the rest of the young ones are able to escape. Zuko, in a change of heart, decides to defy his father and join the Avatar. Zuko catches up with Aang at the Western Air Temple and offers to teach Aang Firebending. After some reluctance from Katara and Sokka, Team Avatar allows Zuko to join the group.
In the four-part series finale, Aang and his friends confront the forces of Fire Lord Ozai, who plans to use the power of Sozin's Comet to destroy the other nations and rule the world as the Phoenix King. Iroh, who escaped from prison during the eclipse, leads the Order of the White Lotus to liberate Ba Sing Se. Sokka, Toph and Kyoshi Warrior Suki take down the Fire Nation's airships, preventing them from burning down the Earth Kingdom. Meanwhile, Zuko and Katara take on Azula. Zuko seems to have the upper hand due to Azula's deteriorating mental state. However, when she fires a lightning bolt at Katara, he takes it to save her, gravely injuring himself in the process. Katara then manages to restrain Azula and heals Zuko. Aang struggles in his battle against Ozai, since he is reluctant to take his life. However, Aang is able to reenter the Avatar State and ultimately defeats Ozai by taking away his firebending powers with a technique known as energy-bending, taught to him by an ancient lion-turtle. With Ozai defeated, the war quickly ends. Zuko is crowned the new Fire Lord and, with the help of the Avatar and his friends, begins rebuilding the four nations. The series ends as Aang and Katara kiss beneath the sunset.
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