Hrothgar is the King of the Danes in southern Denmark. Through success in battle he has become rich and mighty. As a symbol of his power and prosperity he builds a magnificent mead-hall, called Heorot, in which he and his loyal warriors can feast, drink, boast, and listen to the tales of the scops, the Anglo-Saxon bards. But soon after Heorot is finished, the mirth of the men and the music of the scop anger Grendel, a monster descended from Cain. Grendel raids the hall, snatching men and eating them, then returns to his home in the marsh. He repeats his nightly raids until no one dares sleep in the hall. Heorot, once the symbol of the Scyldings' greatness, is now a place of shame and terror. This continues for twelve years, until Beowulf, a young warrior of the Geats in southern Sweden, hears about Grendel and, determined to fight the monster, sails to Hrothgar's lands with fifteen companions.
Hrothgar, who knew Beowulf's father Ecgtheow, accepts Beowulf's offer to fight Grendel and gives him a feast, though Beowulf and Unferth, a warrior loyal to Hrothgar, exchange insults. That night, the warriors sleep in Heorot, with Beowulf keeping watch. Grendel arrives and consumes one of the warriors, then reaches for Beowulf. Beowulf, famous for his powerful grip, which is as strong as the grip of thirty men, struggles with Grendel, tearing off the monster's shoulder and arm. Grendel returns to the fens to die. His arm is hung as a trophy under the roof of Heorot.
Hrothgar gives a second feast to celebrate Beowulf's victory. At the feast, Hrothgar generously rewards Beowulf with treasure. The scop sings again, and Beowulf is praised until nightfall. That night, Grendel's mother comes to the hall from her home in the bottom of a lake, seeking revenge for the death of her son. She grabs Aeschere, a favorite warrior and adviser of Hrothgar's, and consumes him, then returns home. In the morning, the warriors follow her tracks to her lake, where they see Aeschere's head. Beowulf enters the lake, and swims for hours before reaching her cave at the bottom. He fights with Grendel's mother, but the sword Hrunting, which Unferth lent to Beowulf in a sign of fellowship, fails for the first time. From the treasure hoard in the cave Beowulf seizes a sword forged long ago by giants and kills Grendel's mother. He sees Grendel's body, and removes the head, and takes it and the hilt of the giant's sword (the blade melted on contact with the monster's blood) back to Hrothgar.
There is another celebration in Heorot with more gifts and promises of friendship. Hrothgar says he feels like Beowulf is his son, and weeps at Beowulf's departure. Beowulf and his men return to the land of the Geats, where his uncle the king, Hygelac, and Hygelac's queen Hygd, greet Beowulf, accepting the treasure Beowulf gives them, and in turn rewarding Beowulf with a sword, praise, and land. Hygelac is eventually killed by the Swedes; his son Heardred, though young, takes the throne with Beowulf's support. At Heardred's death, Beowulf takes the throne of the Geats, and rules in great prosperity and fame for fifty years.
In Beowulf's old age, a thief finds a passageway into an old barrow. Inside, a dragon guards a treasure trove left there long ago by the last survivor of an extinct people. The thief steals a cup, but the dragon discovers the theft and burns the land, including Beowulf's mead-hall. Beowulf, knowing his death is near, decides to fight the dragon. Accompanied by his kinsman Wiglaf, ten warriors, and the thief, Beowulf sets out to confront the dragon. But when Beowulf and the dragon fight, all of Beowulf's men flee except Wiglaf. With Wiglaf's help Beowulf kills the dragon, but not before he himself is terribly wounded.
Before he dies, Beowulf tells Wiglaf to rule after him, and to build him a funeral barrow that overlooks the sea. Wiglaf chastises the men for abandoning their lord. A messenger sent to tell the Geats of Beowulf's death also warns of hard times for the Geats, now that Beowulf is dead. The Geats build a pyre and cremate Beowulf, then construct a barrow overlooking the sea, burying the dragon's cursed treasure with him.