One hundred years before the start of the series, a 12 year old Airbender Aang learns that he is the new Avatar. Fearful of the heavy responsibilities of stopping an impending world war and with the impending separation from his mentor Monk Gyatso, Aang flees from home on his flying bison Appa. During a fierce storm, they crash into the ocean, and Aang's Avatar state freezes them in a state of suspended animation inside an iceberg.

Aang and Appa are awoken a hundred years later by two siblings of Southern Water Tribe origin, Katara and Sokka. Aang learns that the Fire Nation started a war a hundred years ago, just after his disappearance. The Fire Nation's opening move in its campaign for global conquest was to launch a genocidal attack on the Air Nomads which drove Aang's entire race to extinction, thus making him "the last Airbender" left alive. He realizes that he must fulfill his destiny of becoming the Avatar and return balance to the world by defeating the Fire Nation. Aang sets out to master the three unlearned elements: Water, Earth, and Fire. With Katara and Sokka, Aang decides to head first to the North Pole to find a Waterbending master.

Aang soon discovers that Sozin's Comet, which Fire Lord Sozin used as a power supply to start the war, will return in the coming summer, giving the Fire Nation enough power to ultimately accomplish victory. Aang realizes that he must master all four elements and end the war before this time. For most of their journey to the North Pole, the group is pursued by Zuko, a banished Fire Nation prince and son of Fire Lord Ozai who is obsessed with capturing Aang to restore his lost honor.

After leaving the North Pole and mastering Waterbending, Aang travels to the Earth Kingdom to master Earthbending. There, the group meets Toph, a blind Earthbending prodigy who becomes Aang's second teacher. The heroes discover information about an upcoming solar eclipse which would leave the Fire Nation powerless and open to invasion. They struggle to reach the Earth King with this vital information, but are detoured by Appa's kidnapping. The psychologically self-tormented Zuko, his sister Azula, and her two friends Mai and Ty Lee chase the group as the struggle to reach Ba Sing Se. Azula engineers a coup from within that topples the Earth King and destroys any hope of a large-scale invasion of the Fire Nation.

On the day of the solar eclipse, Aang's group and a smaller band of warriors launch a smaller invasion, which fails. Zuko confronts his father and leaves the Fire Nation. He later manages to gain the trust of the protagonists and becomes Aang's Firebending teacher. Sokka and Zuko then travel to a Fire Nation prison to rescue Sokka's father and Suki, a warrior who befriended the group when they traveled to Kyoshi Island.

On the day of Sozin's Comet's arrival, Fire Lord Ozai starts a genocidial campaign to destroy the rest of the world. Aang and his friends face "Phoenix King" Ozai, Fire Lord Azula, and the Fire Nation army. Aang confronts and defeats Ozai, while Zuko defeats Azula and claims the Fire Nation throne. The series ends as Zuko promises to help rebuild the honour of the Fire Nation, and unite the world again.