One special power that the Avatar has is called the "Avatar State." When an Avatar goes into the Avatar State, he or she gains all of the knowledge and abilities that all of the past Avatars have had. The Avatar State is a defense mechanism. An Avatar goes into this state automatically. However, if he or she practices for a very long time, he or she can control when they go into the Avatar State and what they do while they are in it. If an Avatar is killed in the Avatar State, then the Avatar Cycle will be broken. Then there will be no more Avatars. Through the years, there have been many Avatars. They keep all of the four nations peaceful. Another special power that the Avatar has is being able to go into the Spirit World. He or she can link the physical world to the Spirit World. In this way, he or she can solve problems that regular benders cannot.
One hundred years before the story begins, a twelve-year-old Airbender named Aang learns that he is the new Avatar. He is not happy when he learns this, because he knows that being the Avatar is a big responsibility, and he does not think he is ready for it. He is also scared of having to leave his mentor, Monk Gyatso, whom he loves very much. Aang flees from home on his animal guide, a flying bison named Appa. But they fly into a big storm and crash into the ocean, and Aang goes into the Avatar State and freezes himself and Appa inside an iceberg. They are frozen in a state of suspended animation—they cannot move, grow older, or die. While this is happening, the Fire Nation is beginning a war against the other three nations. Fire Lord Sozin, the leader of the Fire Nation, knows that the new Avatar is an Air Nomad (because of the Avatar Cycle's pattern), so he kills all of the Air Nomads in a genocide. This is because he wants to make sure that the Avatar, who is very powerful, will not be able to defeat the Fire Nation in the war. The FIre Nation army is able to kill all of the Air Nomads because a comet called "Sozin's Comet" made their bending much more powerful. But because the actual Avatar, Aang, is frozen in an iceberg, he is not killed. He is the only Airbender to survive the attack. For one hundred years, everybody in the world believes that the Avatar does not exist anymore.
A hundred years later, Katara and Sokka, two siblings who live in the Southern Water Tribe, free Aang and Appa from the iceberg. Katara is the only waterbender in her small tribe. She has taught herself how to bend. Her older brother, Sokka, cannot bend, but he is a warrior. Katara, Sokka, and Aang travel to the Northern Water Tribe so Aang and Katara can learn waterbending from a master. Aang does not know about the war or the genocide of the Air Nomads. While on their journey, they visit the Southern Air Temple, where Aang used to live. There, Aang discovers that the Fire Nation destroyed the Air Nomads. He also meets his Avatar guide, Avatar Roku, who is the Avatar who came before Aang. Roku tells Aang that he must learn to bend all four elements by the summer solstice. This is because Sozin's Comet will return on that day, and it will make the firebenders' bending very powerful. If Aang does not defeat the current Fire Lord, Ozai, by the summer solstice, then the Fire Nation will take over the world and no one will be able to stop them anymore.
While Katara, Sokka, and Aang travel to the Northern Water Tribe, they are constantly chased by Prince Zuko. Zuko is the son of the current Fire Lord, Ozai, but Ozai banished him from the Fire Nation three years before the story starts. Zuko can only return to the Fire Nation and get back his honor if he captures the Avatar. He travels with his uncle Iroh, nicknamed "Dragon of the West", a legendary Fire Nation general and the older brother of Ozai. Zuko and another Fire Nation military officer, Admiral Zhao, compete to capture the Avatar. At the end of the season, Admiral Zhao leads the Fire Nation navy in attacking the Northern Water Tribe. During the attack, Zhao also kills the Moon Spirit. But Aang and his friends help fight back against this attack, and Yue, the princess of the Northern Water Tribe, sacrifices herself (in other words, she decides to die) so she can become the new Moon Spirit. In the end, Aang and his friends win the battle.
After leaving the North Pole, Aang continues to learn and master waterbending with Katara. Aang, Katara, and Sokka look for someone who can teach Aang how to earthbend. They soon meet Toph Bei Fong, a young girl who is one of the best earthbenders in the world. She is blind, but knows how to "see" (or sense her surroundings) through earthbending and by feeling vibrations, or small shakes, in the earth. She teaches Aang to do the same.
Meanwhile, Zuko and Iroh are now fugitives from the Fire Nation. They escape to Ba Sing Se, the capital city of the Earth Kingdom, where they try to start new lives. Zuko, with the help of his uncle, tries to accept the difficult events he has gone through in his life, and he learns to not be so focused on capturing the Avatar. Aang and his friends (who are sometimes called "Team Avatar") learn about a solar eclipse that will happen in the future. When the eclipse happens, all firebenders will lose their bending for a short time. So Team Avatar makes plans to invade the Fire Nation on that day. This is when Aang plans to defeat Fire Lord Ozai.
While Team Avatar travels to Ba Sing Se so they can tell the Earth King about the eclipse, Zuko's younger sister Azula and her two friends Mai and Ty Lee try to capture them. Finally, the three of them capture the Kyoshi Warriors (friends of Team Avatar) and pretend to be them, so that they can invade Ba Sing Se. 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. At the end of the season, when Team Avatar is fighting against Azula and her allies, Zuko sides with Azula, who promises to restore his honor. When Aang enters the Avatar State, Azula shoots him with lightning. It hurts him very badly, and he almost dies. Katara brings Aang back to life with spirit water that she got from the North Pole in the previous season. But Aang's seventh chakra is blocked, and he cannot enter the Avatar State.
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. 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 who Aang has helped along his journey. Initially the invasion proceeds as planned, but Aang fails to find the Fire Lord before the eclipse ends, because Azula knew about the invasion. The invasion ultimately fails, and only Aang and his closest friends 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 series finale, Aang defeats Fire Lord Ozai by taking away his ability to firebend. 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 in front of the sunset.
Aang (Zach Tyler Eisen) is a 12 year old. He is the most important character in the series. He was frozen in ice with his flying bison, Appa, for 100 years. He was found by a young Waterbender named Katara. Now he is the Avatar, the master of 4 different powers: water, earth, fire and air. Aang is a hero, but he doesn't want to be a hero. He is trying to make the world good again.