Set in the future, on a fictional small earth-like planet inhabited by the na'vi people, called Pandora, the film tells the sotry of Jake Sully (Sam Worthington), a disabled former marine, sent to the planet to take over his dead twin brother's avatar, a man-made human/na'vi hybrid body used to make contact with the indigenous people.
The humans have discovered mineral resources on the planet that they intend to plunder but the na'vi people, who are highly in tune with their ecosystem, are in the way. Sully finds himself stranded with a na'vi tribe and gets to learn their ways and finally becomes one of them. Soon he has to choose between humanity and his adoptive people.
The plot is to a large extent fairly conventional: the hero finds his true self and rebels against a corrupt and oppressive system. Where things are slightly different however for a Hollywood block-buster is in the fact that the humans are the evil group and (warning: slight spoiler ahead) are the ones who lose in the end (which is very open and clearly gives spaces for the two sequels that have alreadry been announced).
It is obviously no coincidence that the planet where the story takes place is named after a character of the Greek mythology famous for releasing all the evils of mankind. Cameron portrays (most) humans as greedy, delusional and warmongering "morons" while the planet and its inhabitants hark back to some primeval Golden Age where senscient beings remain in tune with nature.
Cameron's message against war (I heard people chuckle with me at the famous phrase "we have to fight terror with terror") and in support of a more considered and respectful approach to nature is both quite obvious and rather unusual in this type of film. It is to Cameron's credit and (hopefully) a sign of a change in mentalities that this should happen when Hollywood is known for its rather conservative approach to social and political issues.
From a Lesbian, Gay, Bisexual, Trans (LGBT) perspective, it is also very easy to identify with what Sully is going through. Like most LGBT people, he has to go through a sometimes difficult process of change and learning, finding along the way a new group to which to belong. Then there comes, for some of us anyway, the realisation of the oppression endured by that group and a decision to stand up and fight for the rights of this new "family", sometimes at the risk of betraying old allegiances.
For most people however the main appeal of the film is the use of 3D technology and the world created by Cameron to set his story (He even asked some university professor to create a new language specially). And there is little doubt that the film is indeed visually stunning. The 3D works very well; it is, most of the time, unobtrusive and only very occasionally a bit clumsy.
The Computer-generated imagery (most of the film, I would imaginge) is absolutely flawless (none of this floating impression you usually get) although I have some reservation in the design of some of the creatures that appear in the film. While the Banshees (loosely based on pterodactyls) are convincing, the ground animals are not so much. They seem much too similar to earth animals and the decidion to six legs instead of four to make them look different doesn't make much evolutionary sense.
There are also some improbabilities: Dr. Grace Augustine (Sigourney Weaver)'s smoking in air-tight environments (her first line in the film is to ask for a cigarette, presumably as an easy and lazy way to make her look like a rebel, someone who thinks and acts differently). Another issue is the fact that although Pandora seems mostly covered in lushious rainforest, it doesn't seem to rain there.
Finally, the character's and particularly Sully's change of body happens much too easily. Considering that a na'vi body is about three times as big as a human one, getting used to using such a different set of limbs would probably take much more time than is shown in the film.
But on the whole, Avatar is a hugely enjoyable and entertaining piece of cinema. Despite the length of the film (close to 3 hours), the story, however lacking some may chose to portray it, carries the viewer throughout without a moment of boredom. Highly recommended.
Plus you get to look like this!