Book Four: Air

Avatar: The Last Airbender is over, but was clearly cut short. Here's how Book 4 should have gone:

One year after Fire Lord Zuko's ascension to the throne:

Aang, while living a life of quiet reflection in the Eastern Air Temple, sees a flying bison. This starts him on the search for the missing Air Nomads, who escaped the Fire Nation's wrath by
disappearing beyond the known boundaries of the world. In the last 102 years, they have retreated from earthly concerns, and Aang must convince them to return in order to finalize his life's work of restoring balance. Katara goes with him, despite Aang's insistence that she not.

Ozai bargains with Zuko, offering information on his mother's location in exchange for a 'house arrest' arrangement on the family estate on Ember Island. Zuko, still learning how to be a softy, agrees. Ursa has taken refuge in a murky cave behind a waterfall in the Southern Water Kingdom, where firebending is nearly impossible. The precise location of this place is known only to the Dai Lee, as it is one of a network of hideouts they have established across the globe. Zuko, bored with palace life and a moron, decides to go himself. He appoints Mei as steward in what he believes will be a short absence.

With the Avatar and the Fire Lord now distracted, a group of Ozai loyalists (including the YuuYan archers) takes advantage of the situation and free Ozai. Oops. They embark on a mission to capture the Avatar equivalent of the nuke -- crazy Azula, who has been locked up far from civilization.

Zuko travels to the Ba Sing Se, where Toth has reconciled with her family and set up a school to teach metalbending. He meets with Iroh, who has set up his tea shop and has no interest in adventures. As the Blue Spirit, he sneaks into Dai Lee Central and steals the map of their hidden bases. He realizes they are all linked by narrow passages which can be navigated only by earthbenders. The Earth Kingdom could potentially move people -- or troops -- throughout the world unhindered. Huh. Zuko enlists Toth's help to move speedily to his mother's cave.

A happy reconciliation, and a stop over to see Sokka, who is hard at work rebuilding the Southern Water Kingdom with his father.

Just then, of course, Ozai makes his move. Easily overpowering Mei, he reinstates himself as Fire Lord, assuming that without the Avatar, Zuko cannot get to him. Trouble is, the Earth King has decided that the Fire Kingdom needs to be destroyed to ensure that they never pull a world-dominance stunt again. He unleashes his armies through the tunnel system and battles rage throughout the Fire Kingdom.

Zuko, Toth, and Sokka race back to the action, picking up Suki and the Kyoshi Warriors along the way. They mount a stealth mission to recapture Ozai in hopes that Zuko's reinstatement would calm the Earth King.

All this time, Aang has been searching the hinterlands, proving himself to the Air Nomads, and bickering with Katara. Just when all seems lost back home, he and his people fly in and stop the violence. Zuko reclaims the throne. Everybody goes home.