The movie is a prequel the movie Alien, which is often described as the most scary movie ever made. Actually, the sequel, Aliens, scared me even more. Although Prometheus is not a particularly scary movie, it does present us with a grand vision of the events preceding the first movie along with an interesting mythology to back it up.
The story is that archaeologists discover evidence that ancient humans were visited by an alien race, and they think that they have a good idea where those aliens are from. They convince a corporate trillionaire to finance a huge space ship in an effort to go find the aliens. This is the point where a person might wonder if this is a good business venture? What they find on this trip very neatly leads up to the first Alien movie, which means that if you haven't seen that film, the ending of Prometheus won't have as much significance.
Of particular significance on this trip is an android named David, played creepily by Michael Fassbender. He seems to have his own agenda, and protecting the humans on the ship isn't part of it. Another person who has an agenda, is the corporate trillionaire, played by Guy Pearce. Seems that he is a stowaway on this trip. Charlize Theron plays his daughter, who runs the ship, and she is about as unemotional as the android.
The R rating puzzles me. There is some violence and sexuality in this movie, but neither is very graphic. Compared to other R rated films, this movie seems tame. There is a little bit of alien monster violence that might seem intense.