Throughout the centuries, a mystical being known as the Avatar has kept peace around the world and between its four nations, Air, Water, Earth and Fire. One hundred years ago, however, Aang (NOAH RINGER), the latest in a long line of reincarnated Avatars, ran away from his home among monks when he learned of his destiny. Along with his huge, wingless flying creature, he ended up in suspended animation, forever stuck in the form of a boy, all while the Fire Nation took advantage of his absence and waged war on the other nations.
Knowing that the return of the Avatar would end his reign of terror, Fire Lord Ozai (CLIFF CURTIS) has ordered Commander Zhao (AASIF MANDVI) and his army to find and capture the Avatar in order to keep him at bay. Also on the same quest is Ozai's teenage son, Prince Zuko (DEV PATEL), who can control fire with his mind like his father and others of their nation, but has been banished by Ozai for being too soft. If Zuko, with the help of his Uncle Iroh (SHAUN TOUB), can find the Avatar, his father will lift the teen's exile.
Little do any of them know that teenage siblings Katara (NICOLA PELTZ) and Sokka (JACKSON RATHBONE) have found and rescued Aang, not initially aware of his importance. But when Zhao's forces capture the boy, Katara, who's the last of her "waterbender" kind who can control water through telekinesis, urges her brother that they must save the boy.
After they do and realize he's not only an "airbender," but also the actual Avatar, they set out for the Northern Water tribe where Princess Yue (SEYCHELLE GABRIEL) and others there can help Aang learn how to control the other three natural elements. From then on, it's a race against time to transform Aang into the destined Avatar before Zhao and his forces attack.