Sokka and his sister Kya - later renamed Katara, must travel the world to find masters for Aang, who is the Avatar. However, they must evade a critical foe, Prince Zuko of the Fire Nation, who wants to capture Aang.
The gang arrives at Kyoshi Island and are captured by some Kyoshi warriors. When Aang reveals that he is the avatar, news quickly spreads out and suddenly reaches to Prince Zuko who then goes to Kyoshi Island to capture him.
To contact the spirit of the previous Avatar, Aang must reach a temple in the Fire Nation before the sun sets on the following day. There he will learn what he must do to restore balance to the world. Zuko must defy his banishment to follow Aang.
While Katara herself becomes a great waterbending master, Commander Zhao plans a siege on the Northern Water Tribe. Meanwhile, Sokka's on a mission to stop this siege while Aang tries to buy them some time.
Aang and friends arrive in the Earth Kingdom to begin his earthbending training, but a local general convinces Aang that all he needs to do is learn to control his Avatar state. Azula, Zuko's sister, arrives to escort her brother and uncle back to the Fire Nation.
Still searching for an earthbending teacher, Aang and his friends stumble across a small earth village. When they hear about an underground earthbending tournament, they decide to go to it. Aang finds a teacher that fits the description of King Bumi. Her name is Toph Beifong, a blind, 12 year old girl who is tougher than she looks. When they meet, Toph is at first reluctant, because her parents don't know how good of a bender she is. After her parents discover her secret life, she decides that she wants to join the Avatar gang to help him master earthbending.
Every time they land, Azula and her warriors find Aang, Katara, Sokka, and Toph. Toph refuses to help set up camp, claiming she carries her own weight and no one else's. After a fight between Aang, Katara and Toph, Toph storms off angrily. Aang lays a decoy trail for Azula and Sokka, Katara, Appa, and Momo, flee the other direction. Aang waits for Azula and is fighting her when Zuko and Iroh arrive. Soon, Katara, and Sokka come to the fight, too. Just as all hope of winning is lost, Toph arrives and, together, the six attack Azula. Knowing she is beat, Azula shots Iroh with lightning and disappears. Zuko leaves with Iroh and Aang, Katara, Sokka, and Toph leave as well. Toph and Katara apologize to each other and the journey continues.
After getting tickets for the ferry headed to Ba Sing Se, the gang meets an old friend. They skip the ferry ride to help some refugees, one of which is going to have a baby, get to Ba Sing Se. They must lead the refugees through the Serpent's Pass, a dangerous mountain-side path surrounded by water.
After arriving at the great wall of Ba Sing Se, Aang and the gang discover that the Fire Nation has constructed a great drilling machine to penetrate the wall. The drill is also followed by an army of Fire Nation troops. Also, Zuko and Iroh are having trouble hiding the fact that they are both Firebenders.
After getting a house to stay in, Aang and company are given a tour of Ba Sing Se. As they try to get in to tell the Earth King the news, they find that nothing is quite what is seems. Jet is in the city, too, and he is positive that Iroh and Zuko are Firebenders.
The kids battle Long Feng and the Dai Li when they attempt to show the Earth King the vast conspiracy taking place in his city. Zuko becomes seriously ill and has feverish dreams as he faces a spiritual crisis.
Aang receives guidance from Guru Pathik at the Eastern Air Temple; Sokka is reunited with his long-lost father; Princess Azula has something diabolical in store for her enemies; Uncle and Zuko run their own tea shop.
Weeks after the fall of the Earth Kingdom, Aang and his friends must infiltrate the Fire Nation. Katara meets her father for the first time in years. Zuko returns home, but is uneasy with his newfound reputation.
After deciding he's the least valuable member of the team, Sokka seeks an apprenticeship with a sword-master. The rest of the gang tries to adjust to being without him. The imprisoned Iroh may not be as beaten as he seems.
Zuko makes a decision about his future. Aang reunites with many of his old friends in a full-scale invasion of the Fire Nation capitol to coincide with an eclipse which will remove the Fire Nation's powers.
Zuko has joined up with Aang's group and is now ready to teach Aang fire bending. Unfortunately, Zuko has found out that he has lost his ability and must now relearn it. Zuko and Aang travel to find the original source of fire bending so that they both can learn from The Firebending Masters.
Sozin's Comet is only three days away. Zuko tells the others what the Fire Lord plans on the day of the comet and the gang realizes that they must stop him before then. But Aang doubts that he can do what is expected of him.