Just kidding They really could've penned something a little more original though.
Without giving too much away, you basically choose which side you want to fight as: Either human or Na'vi in a tumultuous moral dilemma. The story, although a prequel to the film, is retarded similar. You play Able Ryder, a dude whose genetic make up is beneficial to the making of a Na'vi Avatar. As far as movie tie-ins go, this is has to be one of the best renditions I've come across. You also have a better understanding of the universe and an appreciation for the Na'vi that can't really be gained by simply playing the game. What are these tall blue people? Why are you turning into one of them? On the other hand, watching the movie first gives the game new appeal as you feel you're contributing in some way to the events that you enjoyed so much in the film. The only problem is I don't suggest you play this before you watch the film as the universe is taken presumption.
The Avatar game is a prequel to the film, setting up the events of what happens in the film. Welcome to Pandora: The vicious but beautiful brainchild of the mighty James Cameron. It's a high-tech, epic blockbuster (check out On Screen's film review) with an awesome universe to play with that potentially lends itself to an awesome game. Not as bad as everyone thinks: The Avatar film has grossed crazy box office figures and has been turned into an instant classic.