After Aang, Zuko, Katara, Sokka, Toph and Suki had defeated Firelord Ozai and stopped the tyranny of the Fire Nation, Aang and Zuko took steps to usher in a new age of equality amongst the nations. In the center of the Fire Nation Colonies, they built Republic City, a place where people of all nations can live without fear of war.
Now 70 years have passed since then, and Aang's successor, Korra, is training to be the new Avatar. After passing her firebending test, she meets with Tenzin, Aang's son and the only Air Bending Master, who is supposed to be her Air Bending Teacher. However, due to tense circumstances in Republic City, he tells her that her training must wait, so that he can tend to his duties in the City. Not wanting to wait, Korra sneaks out of the South Pole and heads to Republic City where she finds herself in over her head.