Main guy is Jake, ex military paraplegic who gets sent to another planet to undertake a mission on behalf of his twin brother (who died before the beginning of the film). An earth base has already been established on this new planet, and the base is overlooked by a facility of scientific researchers, military grunts, and some shoddy business tycoon. Tycoon assigns jake with the mission of gathering intel from the Avatar people so that later the earth dudes can mine and loot the planet for money. Plot is basically a crude version of colonialism, where the Earth people are the colonisers, the Avatars the natives who risk seeing their homes destroyed, and Jake is the saviour, who grows attached to the Avatar (and falls in love with one of them too). Science people support Avatar, military do not, there’s an inevitable clash between the Avatar and the military, with the Avatar winning in the end, although at great cost to their homes. Another important thing to note is that the Earth people are able to assume the body of blank slate Avatars (hence the name) and interact with the others by getting into a capsule and going into a dreamlike state.
main baddie is some colonel dude, but he “dies” in the end, getting impaled with a spear in the chest by Jake while fighting. By the end of the movie, Jake survives the battle, but no longer wanting his human body, asks the shamans to transfer his spirit to an Avatar. From then on Jake no longer exists as a human but as an Avatar, living together with the Avatar chick he hooked up with.