The Big Brawl: Avatar is the sixth installment in The Big Brawl Series. This explains how Aang and co get wrapped up in the whole conflict.
Water, Earth, Fire, Air. Long Ago, the four nations lived in peace. That all changed when the fire nation attacked. Only the avatar, master of all four elements, could stop them, but when the world needed him most, he vanished. 100 years passed and my brother and I discovered the new avatar, an Airbender named Aang. And although his air-bending skills are great, Aang still has a lot to learn before he can save anyone, but I believe, Aang can save the world!
The Fire Nation was not always as peaceful as it is now under the rule of Fire Lord Zuko. The story dates back to the time where Zuko's grandfather was the Firelord, or even further, his ancestor Sozin.