What is Avatar actually about?

I’ve been seeing a flood of advertising and promotion for this movie for the past 3 months, dating back to the World Series and earlier. One of the shows I watch (Bones) was even made into a commercial for it a couple weeks back. And yet with all that hype, I still have no clue what the movie is about. What is the plot?!

All I’ve heard from any of the advertising, marketing, and reviews is that the visuals are amazing. That it changes the way CGI movies will be made from now on. Not a single piece of promotion has told me anything about whether or not this movie has an actual story I’d care about. I get that there’s a marine, virtual reality, and aliens, and some interaction involving the three of them. But beyond that, not a friggin clue.

So can someone please indicate to me whether this is a movie I’d have any reason to care about if not for the advances in technology? Any quick synopsis of the plot of the movie?