Based on the epic poem, the film is about a man who comes to Heorot in order to rid it of Grendel, the monstrosity who terrorizes it. What he does not expect is Grendel's Mother.
While most of the film remains true to the original poem, other elements, such as the dialogue, the emphasized hubris of Beowulf and Hrothgar, the perceived hedonism in Heorot, the span of time before Beowulf fights the dragon, Grendel's motive for attacking Heorot and the role of Grendel's mother seem to deviate from the original poem.
i just realised this movie got my beloved J.R.R. Tolkien related 1
The Poem:
http://en.wikipedia.org/wiki/Beowulf