From what I can understand at this point is that this may be the first of three movies.
It really doesn't give much background before the movie begins. What I learned is from the site above.
I think the plot is that Riddick is from an almost extinct race where they are the only one that can kill the leader of the Necoromongers. Riddick has been eluding mercenaries for five years because he has a bounty on him for previous crimes. He is brought to an old friend who knows he is the only one that can save his planet and the galaxy from the Necoromongers. This he does and ends up becoming their leader in the process.
Hope that helps
