For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Grendel introduces his plight to us: he's been warring against Hrothgar and Co. for twelve years. He steps back to tell the story from the beginning (with a few intrusions from present time). We see Grendel encounter the human world in painful ways—both physically (an ax through the shoulder) and mentally (the beautiful but accusatory songs of the Shaper).
Freaked out by the Shaper's song and pretty much in total despair, Grendel comes into contact with the dragon, and his choices are cemented. Act I concludes the moment Grendel, who's been attacked by Hrothgar's guard, makes the decision to raid the meadhall.
Act II
It's the winter of Grendel's (and everyone else's) discontent. Hygmod arrives and squares off with Hrothgar, losing his sister Wealtheow to him in the process. Grendel is tormented by Wealtheow's beauty and her sad plight, and he can't bear to attack Hart for a while.
Hrothulf also becomes a member of Hrothgar's household, and tensions rise. Toward the end of this act, the Shaper gets sick and dies. Grendel watches the end of an era as the smoke rises from the Shaper's funeral pyre. He feels strangely abandoned.
Act III
Grendel mopes about the countryside for a bit, picking up whispers and rumors about a giant from overseas who has the strength of 30 men. He dismisses it at first but feels something strange and frightening in the air. Plus, his mother has become insane and completely withdrawn.
Soon, the Stranger (yep, it's Beowulf) arrives at the seacoast with his weird band of Geat dudes. Grendel approaches the meadhall to scope out the competition and gauge the tension between the Danes and the Geats. This act ends with the clash of titans, so to speak. Grendel teeters on the edge of death before he finally buys the farm.