This movie is kind of a mess. Based on the books by Edgar Rice Burroughs (who also wrote Tarzan), its about a former Civil War captain who enters a cave, is transported to Mars (known by the inhabitants as Barsoom) and gets involved in wars and battles.
I have never read the books, but from the research I have done, it seems as though the filmmakers took a simple fantasy / sci fi adventure and made it as complicated as they could. They give Carter a backstory, and it takes way too long for him to get to Mars. It seemed like 20 - 30 minutes had gone by before he found that cave.
Once there, he finds that he has powers. Thanks to the different gravity, he can jump like 80 feet with ease. He also seems to be really strong, although the movie isn't consistent about that. One moment he is fighting an entire army off, the next he is held in chains.
He meets a tribe of giant green creatures with 4 arms. They don't trust him at first, then they are amazed by his leaping ability. A little later he meets the human looking people of Mars. He falls in love with a princess and gets involved in some kind of plot to force her to marry her bitter enemy.
There are also characters called Therns. They weren't in the first book, they popped up later in the series, but the filmmakers put them into this movie because there just weren't enough characters or something. It just isn't confusing enough - let's add the Therns from book 5!
Anyway, the Therns can shape shift, they seem to be immortal, they have visited other planets, and they are the reason Carter was transported to Mars. They have some kind of plan to kill everyone on Mars, then Earth was their next target, but I may be mixed up on that.
The movie is too confusing. I had a hard time caring about John Carter, the princess, or anyone else in the movie. The reason movies like Star Wars and Lord of the Rings work so well is they have well developed characters we care about, and the story isn't that complicated. There may be complicated things going on around them and many other characters involved, but the main plot is pretty simple. Also, casting a better, more charasmatic actor as John Carter may have helped.
Oh, and the movie is over 2 hours long. It should have been about 90 minutes. I think that's it for now.