Avatar Book 2: Earth

After leaving the North Pole and Aang finishes mastering Waterbending under the instruction of Katara, who has mastered the element faster than he. Aang and friends travel to the Earth Kingdom to master Earthbending. Their initial plan was to learn under Bumi, king of the Earth Kingdom city of Omashu, but discover that he and Omashu have been captured by the Fire Nation and Bumi is waiting for the right time to strike. Searching for a new Earthbending teacher, the group meets Toph, a blind Earthbending prodigy who becomes Aang’s second teacher. The heroes discover information about an upcoming solar eclipse which would leave the Fire Nation powerless and open to invasion. They struggle to reach the Earth King with this vital information, but are detoured by Appa’s kidnapping. Azula, Zuko’s sister, and her two friends Mai and Ty Lee chase the group as they struggle to reach Ba Sing Se, the Earth Kingdom capital. Azula engineers a plan that allows the Fire Nation complete control over Ba Sing Se and the rest of the Earth Kingdom, and destroys any hope of a large-scale invasion of the Fire Nation. Azula is believed to have killed the Avatar using lightning while Aang was in the Avatar state, and there is no hope left. In reality she disaligned his seventh chakra, thus blocking the Avatar state.

Zuko deals with internal conflict as he and Iroh secede from the Fire Nation, settle in the Earth Kingdom, and disguise themselves as refugees. After a brief period of reformation, Zuko helps Azula defeat Aang and is allowed to return back to the Fire nation as a re-instated prince.

SOURCE: http://en.wikipedia.org/wiki/