The fantasy cartoon "Avatar: The Last Airbender" was one of the most popular and critically acclaimed shows on Viacom Inc.'s Nickelodeon network. Now the creators of the show are back with a sequel series called "The Legend of Korra" that they hope will expand the audience for the program despite a change in the title, a switch in the setting, and a new female protagonist.

"Avatar: The Last Airbender" was set in an imaginary world in which some individuals, called benders, have the supernatural ability to control water, earth,...