The movie begins, as with the book, with a young Edgar Rice Burroughs being summoned to his rich Uncle Carter's Estate. He is told his uncle is dead and that as per his request is in a crypt that can only be opened from the inside. He's also given a diary, which explains everything--
The movie then goes to right after the Civil War, with Carter out West. He's in a saloon/store when he's "recruited" by some Army guys to help fight the Apaches. He doesn't want to fight anymore, which has to do with what happened to him in the war. He escapes, they follow and there's a brief battle between Americans and Indians. Carter takes refuge in a weird cave where he's miraculously transported to "Barsoom", as the natives call it. He finds that he is far stronger and it takes a bit before he can even walk--and then comes upon a hatchery of the four-armed, green Tharks. Tars Tarkas, the leader or Jeddak, is fascinated with him and takes him back to their camp. It's here he meets princess Dejah Thoris, who has fled her city of Hellium because she doesn't want to marry the ruler of Zodanga, their mortal enemy. In order to same Hellium from destruction, her father has promised her in marriage to the evil ruler. This guy also has the help from the mysterious Therns, white bald guys who have given him a powerful weapon called "The Ninth Ray". These Therns can assume the shape of anyone.
While, for the most part, the spirit of the books is captured, there are too many unnecessary changes, most notably with the Therns being aliens and using Mars and Earth as "host" worlds (in the books they are simply another race of Martians). It sort of made the story seem like Philip Jose Farmer's RIVERWORLD and took some of the "oomph" of Carter exploring an alien world. Also, the Tharks aren't quite harsh enough and the casting of the two leads is sort of off the mark. But the visuals are amazing, from the dog-like Woola (I want a calot as a pet!) to the flying machines to the giant white apes (except they are blind here, which makes no sense).
Overall, this is an "okay/good" adaptation of the Edgar Rice Burrough's novels. If it gets a new generation of people to read the original series, all the better.