The poem starts out by acquainting us with the dire predicament in which Hrothgar, the king of the Danes, finds himself. His great hall, Heorot, has been terrorized by a monster named Grendel for a number of years. This creature makes frequent night attacks on the Hall, killing many of Hrothgar's men and- eww- eating them. Hrothgar was once a mighty warrior, but is elderly now, and seems helpless to do anything to put a stop to Grendel's snacking habits.
One day, a thief sneaks into the lair of a dragon and steals a goblet. The enraged dragon storms forth, attacking the villagers, burning their homes, and generally killing everyone he comes across. Of course, Beowulf isn't about to let this go on, so he heads out to face the dragon, accompanied by eleven of his warriors and the thief, who is to show them the way to the dragon's lair. At the sight of the monstrous dragon, the warriors all run away in terror, leaving Beowulf to face him alone. Only one of them, Wiglaf, conquers his fear and returns to help. He finds his king engaged in a fierce battle with the creature, and leaps to his aid. Together, they manage to vanquish the dragon; Beowulf finishes it off, but he himself is fatally wounded. As Beowulf dies, he has Wiglaf pile up some of the dragon's wealth so he can see it while he expires. Each to their own.