For the record I read the premise on this somewhere and it went pretty much as follows.
In the distant past the dragons were destroyed and buried with only a magic spell (or something of that nature) left to bring them back. Naturally someone stumbles on the spell long after the world has forgotten about it and sets the dragons loose again. Fast forward a few years and the dragons have taken over and humanity is being wiped out. One sherrif and one "cocky" American team up to take back the world.
The premise is hard to explain without sounding a little hocky but I saw the preview at the theatre and it did seem badass