"The Legend of Korra" is an animated series that runs on Nickelodeon, but it's more than a kids' show: created by the team behind the "Avatar: The Last Airbender," "Korra" is a sequel of sorts that in its first season has become one of the most popular animated programs on TV.

Set in the same fictional world as the "Avatar" series, "The Legend of Korra" is populated by characters who can control, or "bend," the elements of earth, air, fire or water. Only one, known...