I read the screenplay for this about 5 years ago at this point. Honestly surprised it took so long to get made. I guess Cameron wanted technology to catch up to his imagination.
The basic plot is a) humanity discovers alien world, b) populated by weird creatures, the most intelligent of which are the blue humanoid ones - you can think of them sort of as Native Americans; and c) that's pretty much the plot. Humans come to conquer this new world.
To do this they d) grow "avatars" who are biologically like the aliens but into which human consciousness can be uploaded. This is done to e) interact with the aliens and convince them to let humans take over their planet. f) The aliens are not so keen on the idea and g) fight back. h) it turns out that they're not just primitives but that they i) live in close consciousness-sharing harmony with other creatures on their planet and j) their entire planet via plants.
See, they k) have nerve bundles growing in their hair and these let them connect with other living beings, such as l) the pterodactyls which they're able to pilot by mind control. m) One particularly nasty human soldier scalps one of the main aliens and this is a very dramatic thing. n) the protagonist is a crippled Earth scientist who can't walk, but when loaded into his "avatar" he can, and so he wants to stay in his alien body. o) When an alien dies they get absobed by the foliage and become part of the planetary consciousness. p) Because the protagonist helps to chase off the nasty humans q)by wiping out the invading force and sending Earth a fake message about a lethal virus being on the planet, r) the aliens make him a permanent alien. s) there is also the obligatory love story.
There, aren't you glad I just saved you 2 hours and $20???
Sure, it'll be visually pretty but the plot is lame. Unless you're 13.