If the movie is adapted from the anime series Avatar The Legend Of Aang,i have seen every episode of this anime and loved it.Aang a wind bender and only a young boy raised by monks was chosen as the next Avatar,to learn all 4 bending element,in order to defeat the evil fire lord king,who what else-wants to rule the earth!

Scared&confused he ran away with his flying baison,they got frozen in a huge piece of ice.
100 years or so later,a brother&sister[she a water bender] discovered and released him.The fire king had wiped out all the wind benders in the world,he's the only one survivor & now must learn the other elements fire,earth&watar to defeat the fire lord&his army from wiping out other bending nations using meteors which will give him great supernatural powers.Along the way,the fire lord's son who was determined to capture Aang at all cost to gain his father love,become his fire bender teacher.
In the end,the fire lord defeated&prisoned.His son became the new fire lord promising peace between all nation.Even Aang found love in the water bender who saved him from his icy prison.
This is just a brief sum up of the series.If Cameron is making the movie than it's not going to be just one but a series.