Avatar - The Last Airbender: Into the Inferno follows the show's third and final season,
Book Three: Fire, and has Aang attempting to learn firebending (from the most unlikely of teachers) in order to stop the Fire Lord before a comet passes by and causes all of the firebenders in the world to become unstoppable for a short time.
While the game falls short in quite a few places, Into the Inferno does seem to do a fair job sticking to the series' story, which is good since this is a very eventful and climactic part of the Avatar tale.
For the most part, gameplay takes on the form of an action/platformer where you will be going through linear level after linear level, taking out waves of firebenders. You will be able to play as Aang, Katara, Sokka, Toph and even Zuko, and switch between them at will (once the characters become available, of course). Each one has their own abilities, like Katar's waterbending and Toph's earthbending, but seeing as Aang can do both of those, along with his airbending, and eventually firebending, there becomes little purpose to switch off of him.
One of the effects I actually really liked was the level map. Fans of the show will recognize the world map to be the same stylized view of the world as seen in the series' opening sequence, but now it is rendered in a more 3D fashion, and a little Appa with Aang on top floats above it as you fly from location to location.
Besides the level locations, you can also use the map to visit Ember Island. Here you can use the money you earn in levels to buy everything from artwork to new abilities. There are even a couple of mini-games that will have Aang fly around the island in order to earn more money and unlock more things to buy.