Transylvania is ruled by the charming and likeable Vlad the Impaler who is a Good King(TM) and has a wife he worships and a son he adores. Yup, that's how they're playing it. Vlad is Transylvanian, but he was a child tribute to the Turks, a former Turkish child soldier (very good at it too). And now the Turks are demanding another tribute of young boys as soldiers, including Vlad's son. His wife sobs and cries and reminds him he promised her this would never happen ... so he kills the emissaries and brings the wrath of the entire Turkish empire down on his small country.
In an attempt to defend his family (and, incidentally, his country), he goes to that cave on Broke Tooth Mountain, where he makes the deal and becomes a vampire. Temporarily, he hopes.
Warning, Spoilers: This is meant to be a great tragedy, at least the ending sort of essays that. He slaughters tens of thousands of Turks, but for some reason their leader uses a sneaky flanking maneuver that should have been totally unnecessary because Transylvania had no army - at all. And the Turks did this not even knowing what Vlad had become - why? Halfway through the movie the Turks kill everyone in his entire country, but this is as nothing to the deaths of two or three people near him. It doesn't play well.
Finally, they wrap up with a noble and tragic ending ... which they proceed to completely sabotage and de-fang by a questionable technicality and extending the movie to the modern day to leave room for the inevitable (they hope) sequel.
The movie is supported by some brilliantly applied special effects work that's wonderful to see, but the plot is so bad it's not worth watching at all.