King Hrothgar, the ruler of the Danes, is troubled by the rampages of a demon named Grendel. Every night, Grendel attacks King Hrothgar's wealthy mead-hall, Heorot, killing Danish warriors and sometimes even eating them. Hrothgar was a great warrior in his time, but now he's an old king and can't seem to protect his people. Fortunately, a young Geat warrior named Beowulf travels to Heorot Hall from his own lands overseas to lend a helping hand – literally.
After explaining that he owes Hrothgar a favor because Hrothgar helped out his father, Beowulf offers to fight Grendel himself. King Hrothgar gratefully accepts his offer. The next time Grendel attacks Heorot Hall, Beowulf is waiting for him. Choosing to fight Grendel in hand-to-hand combat, Beowulf wrestles the demon into submission and eventually tears off his arm at the shoulder. Mortally wounded, Grendel flees into the wilderness and dies. Beowulf, Hrothgar, and their followers throw a wild party to celebrate. Hrothgar also gives Beowulf many presents and treasures to reward him for his heroic defeat of the demon.
Unfortunately, Grendel has an overprotective mother who decides to avenge her son. While all the warriors are sleeping off the party, she attacks Heorot Hall. But when the warriors wake up, she panics and flees back to her lair, a cave underneath a nearby lake.
Beowulf, his Geatish warriors, and some of Hrothgar's Danish warriors track her there. Beowulf dives into the lake and finds the cave, where he takes on Grendel's mother in another one-on-one battle. Seizing a nearby sword from Grendel's mother's stash of treasure, he slays her, even though her poisonous demon blood melts the blade. When Beowulf returns to the surface, carrying the sword hilt and Grendel's severed head, the Danish warriors have given him up for dead, but his own Geatish followers are still waiting patiently. When everyone sees that Beowulf has survived this second challenge, there's even more partying and gift-giving.
Finally, the Geats take their leave of the Danes; Beowulf says goodbye to King Hrothgar and sails back to Geatland, where he is a lord in the court of King Hygelac. Eventually, Hygelac and all his relatives are killed in different blood-feuds, and Beowulf becomes the King of the Geats. Beowulf reigns as king for fifty years, protecting the Geats from all the other tribes around them, especially the Swedes. He is an honorable and heroic warrior-king, rewarding his loyal thanes (warrior lords) and taking care of his people.
But one day, Beowulf finally meets his match: a dragon, woken by a thief stealing a goblet, begins attacking the Geats, burning villages and slaughtering people. Beowulf takes a group of eleven trusty warriors, plus the thief who knows where the dragon's lair is, to the barrow for a final showdown with the monster. When they see the dragon, all but one of the warriors flee in terror. Only one man, Wiglaf, remains at Beowulf's side. With Wiglaf's help and encouragement, Beowulf is able to defeat the dragon, but he is mortally wounded in the process.
After Beowulf's death, the Geats build an enormous funeral pyre for him, heaped with treasures. Once the pyre has burned down, they spend ten days building an enormous barrow (a large mound of earth filled with treasure) as a monument to their lost king.