Alright, so I've already posted the Story Line in the Getting Started topic, but I'll repeat it here... perhaps add a little more information.
After Aang defeated Ozai with energybending, he is put behind bars, losing his position of Fire Lord. Zuko, heir to the throne is made the new Fire Lord. He ends the war, and the four nations are at peace once again.
Slowly but surely, known faces start to pass away due to old age, and new people appear to fill in the empty spots. A new Fire Lord, a new Earth King.. and a new Avatar, born in one of the two Water Tribes.
What will happen to the World of Avatar?
You decide.