Earth... Fire... Air... Water. Only the Avatar can master all four elements, and bring balance to the world. But in lieu of balance in the world, feel free to listen to this quick podcast on The Legend of Korra, which covers the entirety of Book One and the first two episodes of Book Two, "Rebel Spirit" and "The Southern Lights." Be warned, this podcast is a master spoilerbender, so if you haven't seen the show yet then by all means catch the hell up then listen.
Tim's TV Talk Podcast, Episode 8: Korracast