You remember Avatar: the Last Airbender, right? Sure you do.
If you haven't managed to see it, it combined a fascinating new world, compelling characters, great animation and a complex story involving martial artists who huck huge amounts of elements at each other. What's not to like? Go watch it immediately.
Then a live-action movie came out. It was awful. Seriously. You're better off not asking about it.
The only plus side to the movie is that Nickelodeon decided to give the original creators of the show the chance to do a sequel series, with a bigger budget no less. And so The Legend of Korra was born.
Set 70-odd years after the events of the first series, the new show focuses on Korra, the new Avatar after Aang. A native waterbender, Korra quickly mastered earthbending and firebending, though airbending took more time. Though she's cheerful and determined, she's much more likely than Aang to approach a problem by punching it to death. She also has a huge polar bear dog named Naga. (Yes, every animal in the world is, er, two animals. Just go with it.)
To learn airbending, she travels to Republic City, a thriving metropolis founded by Aang and Zuko after the events of the first show. It's technologically advanced -- think 1920s Hong Kong -- and a utopia for all four kinds of benders and non-benders alike. Er, that's the theory, anyway.
Her airbending teacher, and continuing mentor through the series, is Tenzin, one of Aang's sons. He's patient, focused and voiced by the awesome J.K. Simmons. His kids are Jinora, Ikki, Meelo and Rohan who are... well, less patient and focused. Jinora now has a strong connection to the spirit world, while Meelo has somehow become a badass drill sergent.
Along the way, Korra meets Mako and Bolin, brothers who grew up on the streets. Mako, a firebender, is stoic and reserved, while Bolin, an earthbender, is goofy and excitable. Though Korra, Mako and Bolin competed in pro-bending as the Fire Ferrets (named after Pabu, Bolin's fire ferret), they've since left the team. Mako is now an on-again, off-again cop in Republic City, while Bolin has starred in some hilariously awful movies. Both brothers are pretty crap at dating.
Lin Beifong is Toph's daughter, only she's less blind and more of a hardass. She leads Republic City's police force, who are metalbenders to the man. She's become even more of a jerk in book 3, especially toward her sister Suyin Beifong, leader of the metal city of Zaofu. What's up with that?
Asami Sato, heir to motor company Future Industries, joins Team Avatar mid-way through book one and has stuck around ever since. She's not a bender, though she's still formidable in combat with her electrified gloves.
Billionaire Varrick is fantastic. That's all you need to know. His assistant, Zhu Li, frequently does the thing.
If you're just catching up with the series, book one had Korra against Amon and the Equalists, a group that wanted to rid the world of bending. Book two had Korra exploring the spiritual world with her uncle, Unalaq. That book also told the tale of the first Avatar, Wan.
As for book three, going on now? Well...
This seems to be dealing with the fallout of Korra's decision to open the spirit portals, especially the side effect of new airbenders popping up everywhere. Team Avatar sets out to help rebuild Airbending society. Meanwhile Zaheer, voiced by Henry Rollins, is revealed to be the reason Korra was in hiding for so long. He gains airbending, and breaks out a team of super-powerful benders, including Gahzan, an earthbender who can superheat rocks, Ming-Hua, an armless waterbender who attacks with water tentacles, and P'li, another firebender who can cause explosions with her mind. The fans have taken to calling them Team Anti-Avatar.
Books one and two are out on DVD and Blu-ray, and all the episodes are available for streaming or digital download. The fourth and final season is already in production.