Season One (Book One: Water)
After being frozen in an iceberg, Aang and Appa are awoken a hundred years later by two siblings of the Southern Water Tribe, Katara and Sokka. Aang learns that the Fire Nation started a war a hundred years ago, just after his disappearance. The Fire Nation launched a genocidal attack on the Air Nomads, starting the war and driving Aang's entire nation to extinction making him "The Last Airbender". He realizes that he must fulfill his destiny of becoming the Avatar and return the balance to the world by defeating the Fire Nation army. Aang sets out to master the other three elements: Water, Earth, and Fire. With Katara and Sokka, Aang decides to head to the North Pole to find a Waterbending master for himself and Katara, the only Waterbender in the South Pole. The journey is long, made longer by Aang's lack of focus and desire for fun and discovery. During the journey however, the group manages to save the lives of many people, several of whom will become future allies, and Aang and Katara gain strength in waterbending.
During one brief stop over, Aang finds his way to the spirit world where he encounters Avatar Roku's dragon animal guide, who instructs him to travel on the day of Winter Solstice to the Fire-Temple on Crescent Island and speak with Roku. Roku tells Aang that he must master all four elements and end the war before the next summer, as Sozin's Comet will be closest to the world then in its hundred year orbit. Fire Lord Ozai will end the war using this comet, an additional power source that will make the fire benders even stronger. The comet was named after Ozai's Grandfather, Sozin, who also used the coming of the comet to start the war, and deal a deadly first strike to the other nations.
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, and by Commander Zhao, an ambitious naval officer who also wants to capture the Avatar to further his own ambitions. Zuko is obsessed with capturing Aang to restore his honor and his place in line for the Fire Nation throne; he was banished for speaking out of turn in a war meeting and refusing to fight his father in an agni kai (literally a 'fire duel'). Zuko was accompanied by his uncle,General Iroh, a wise and kindly general who was supposed to be the successor to the Fire Nation throne before it was usurped by his younger brother Ozai. Fire Lord Azulon, the father of Iroh and Ozai and the son of Sozin approved of this because Iroh's only son, Liuten, was killed in battle, leaving no one as a possible successor. Ozai's children, Azula (named after Azulon) and Zuko were alive and well. Zuko and Iroh were the first to discover that the Avatar is still alive, but Zhao, after capturing Zuko's ship and questioning its crew, also learns of the Avatars reappearance. Zhao prevents Zuko and Iroh from pursuing the Avatar while starting a search of his own. The Fire Lord, seeing Zhao's zeal in the pursuit, promotes the commander to Admiral status, making him in charge of the operation to capture the Avatar.
When the Avatar reaches the Northern Water Tribe along with Sokka and Katara, he and Katara are trained in the art of waterbending by Master Pakku. When the Fire Nation attacks the North Pole, Aang enters the Avatar state and destroys their fleets of ships after restoring balance to the stolen Moon Goddess. The Season ends with the group having destroyed Zhao's attempts at conquering the North Pole.
Source: http://en.wikipedia.org/wiki/Avatar:_The_Last_Airbender
