"Enter an unseen world in James Cameron's Avatar: The Game, the official video game based on the Epic Action Adventure movie. Journey deep into the heart of Pandora and ignite the ultimate war on an alien planet beyond imagination." Back of game case
Set in 2152 (two years before the events of the film), Avatar: The Game starts out with a new Sig Spec arriving named Able Ryder, whose first mission is to save five marines from viperwolves. After saving the marines Ryder must go help another Sig Spec who is afraid of the viperwolves. Then Ryder gets on a boat to another base, where Ryder is told to go fix the fences. After fixing the fences Ryder gets told to get in his avatar. Ryder's first mission in his avatar body is to get cell samples. After getting the samples a Na'vi tells Ryder to kill his infected animals. Ryder finds out that there is a mole and that it is one of the avatar drivers. He must then follow a Na'vi who will lead him to the mole. After finding who the mole is Ryder must make a decision of killing him or siding with the Na'vi by wiping out the marines. (PS3 and XBOX 360 versions; Wii version completely different.)
Source: Wikipedia,James Cameron's Avatar: The Game", available under the CC-BY-SA License.