Episodes
There are three seasons of the show, called Books. Each Book follows the Gaang through their journey to defeat the Fire Nation while Aang learns to master the four elements.
Book One: Water
In the season finale "The Siege of the North", the Fire Nation has followed Aang to the Northern Water Tribe and are preparing to attack the heavily fortified city. Aang believes that if he travels to the spirit world and contacts the moon and ocean spirits, he can defeat the Fire Nation armanda. The army is successful at breaching the Water Tribe's ice wall and begin ransacking the city. Admiral Zhao infiltrates the Spirit Oasis and kills the mortal form of the moon spirit, throwing the world off balance and taking away the waterbenders powers. Princess Yue scarifies herself to serve as the moon spirits mortal coil, since she had some of its spirit imbued in her when she was a sick child. Yue leaves the mortal world for the spirit world and balance is restored and the Water Tribe is able to push back the invaders.
Book Two: Earth
In the chapter "The Earth King", Zuko deals with the consequences of rescuing Appa from the secret Earth Kingdom police, the Dai Li. Uncle Iroh is proud of Zuko, because he made the right choice, thus setting him on a path to good. But the conflicting decision causes Zuko to become ill. He hallucinates that he is the Fire Lord, with two dragons twisting behind him on the throne. the blue one speaks in Azula's voice, telling him to rest. the red dragon tells him to get out. Zuko is in conflict with himself: the person he has always been, who is mean, cruel, and set on capturing the Avatar; and the person who loves his uncle and loved his mother, who saved Appa. Iroh sees this as a "metamorphoses", and that Zuko will come out on the other side as the "beautiful prince" he was meant to be. When he awakes, he goes to the sink to wash his face, only to feel that his scar isn't there. In a panic, he looks in the mirror only to see himself with no scar and a blue arrow on his forehead like an Air Nomad. He wakes up screaming.
Book Three: Fire
In the four-part finale of the show, Sozin's Comet, the final battle begins. Aang is struggling to come to terms with the fact that he has to kill Fire Lord Ozai in order to stop to Fire Nations reign of terror and the conquest of the Earth Kingdom. He consults all his past lives who tell him the same thing, he has to kill Ozai. This goes against everything he believes in as an Air Nomad. In a dream he is lead to a mysterious island that holds the answer to defeating Ozai without killing him. The rest of the Gaang and their allies that they have collected throughout the year prepare for battle. At the climax of the battle between Ozai and Aang, Aang goes into the Avatar State, which connects him to all his past lives and their power. Right when he is about to deliver the killing blow to Ozai, he stops. On the island a turtlelion tells him that there is a way to bend a persons energy, since that is an element. When Aang bends Ozai's energy, he is able to take away his bending, thus eliminating the threat without killing him. Aang found a peaceful solution to the violent conflict with the Fire Nation.
Sources: Avatar Wiki. Retrieved from http://avatar.wikia.com/wiki/Avatar_Wiki.
Dante DiMartino, M.,Bryan Konietzko, Aaron Ehasz (Producers). MacMulan, l. Dave Filoni. (Directors). (December 2, 2005). "The siege of the North Part 1-2" Avatar: The Last Airbender. Nickelodeon, DR Movie, JM Animation.
Dante DiMartino, M.,Bryan Konietzko, Aaron Ehasz (Producers). Spaulding, E. (Director). (December 1, 2006) "The Earth King." Avatar: The Last Airbender. Nickelodeon, DR Movie.
Dante DiMartino, M.,Bryan Konietzko, Aaron Ehasz (Producers). Spaulding, E. Giancarlo Volpe, Joaquim Dos Santos. (Directors). (July 19, 2008). "Sozin's Comet, Part 1-4". Nickelodeon, JM Amination, Moi Animation.