Season Three (Book Three: Fire)
Ba Sing Se has fallen, and Aang is critically injured. He awakes to find his group on a Fire Nation ship disguised in similar regalia.
Sokka has planned a small-scale invasion of the Fire Nation to claim the palace and ultimately defeat Lord Ozai, making use of a soon-approaching solar eclipse which will render Firebending impossible for eight minutes. They travel to the rendezvous point to meet with various warriors that the group encountered in Book One and Two.
The invasion proceeds as planned with the group defeating much of the land forces and watch towers, but Aang finds the palace empty; the royal company has taken refuge in a nearby volcanic temple and prepared an air fleet in a surprise counter-maneuver. Aang never finds Lord Ozai, but Zuko does and uses the eclipse as his chance to confront his father and proclaim his decision to join the Avatar. The eclipse is over, and the air fleet destroys the protagonist party's ships. Only Aang's sky bison Appa can provide a means of escape, and Aang retreats with the children aboard Appa while the adults are taken captive.
Zuko catches up with Aang at the Western Air Temple and offers to teach Aang the art of firebending. After some conflict, he is allowed to join the group. The two travel to unlock the firebending secrets of the "Sun Warriors". He and Sokka also plan an infiltration of Boiling Rock, the prison where high ranking prisoners of war are kept. They find Sokka's father and Suki, the leader of the Kyoshi Warriors and Sokka's former love interest, but Zuko is quickly discovered and Azula comes to question him, accompanied by Mai and Ty Lee. Both girls eventually aid Zuko and Sokka in escaping the prison, betraying Azula in the process. Enraged, Azula demands their imprisonment and begins to slip into paranoia and insanity at the loss of her most trusted confidants.
After regrouping, Sokka tries to create a battle plan, but peace-loving Aang refuses to kill any living being, including Lord Ozai. He is drawn to a mysterious island and meditates on other ways to defeat Ozai by calling on the spirits of other past Avatars for advice. Each Avatar advises him to destroy Ozai, and Aang is reluctantly resigned to killing him until he discovers that the island is really an ancient, giant, wise "lion-turtle." Aang asks it for advice, and the Lion-Turtle teaches him the ancient art of Energybending as a way to avoid killing Ozai.
On the day of Sozin's Comet, Lord Ozai, now the self-proclaimed "Phoenix King" harnesses the comet's power to start a genocidal campaign to destroy the rest of the world. Aang's friends (Aang still missing from his trip to the mysterious island) set off to face him, Azula (soon to be crowned the new Fire Lord), and the entire Fire Nation army. Zuko and Katara go to face Azula just moments before her coronation, so that he can usurp the throne in her stead; Sokka, Suki and Toph fight the air fleet; and the Order of the White Lotus fights to reclaim Ba Sing Se from the Fire Nation. Aang returns just in time to take on Lord Ozai. Finally reclaiming his ability to enter the Avatar State, Aang almost kills Ozai before restraining himself and instead calls upon the lost art of Energybending to remove Ozai's bending abilities altogether. The war over, Zuko is crowned Fire Lord, and alongside Aang, he promises to bring harmony to the entire world.
Source: http://en.wikipedia.org/wiki/Avatar:_The_Last_Airbender
