Beowulf opens with a description of the origin and history of the Scylding dynasty, tracing its descent down to Hrothgar, who builds Heorot, a great hall. But the sounds of rejoicing anger a monster, Grendel, and one night he kills thirty sleeping men. These attacks recur for twelve years, during which no man can safely sleep in the hall. Beowulf, nephew of Hygelac, king of the Geats, resolves to come to the aid of Hrothgar. He sails with fourteen warriors and, despite initial suspicion, is welcomed to Heorot. Hrothgar accepts Beowulf's offer to cleanse Heorot. The Danes feast with the visiting Geats and Beowulf's bravery is questioned by a Danish kinsmen, Unferth. Beowulf promises to conquer or die.
That night, the Danes and Beowulf's men fall asleep. Beowulf alone is vigilant. Grendel arrives at Heorot, devours a Danish warrior, and seizes Beowulf. The two wrestle bitterly. Beowulf eventually tears off Grendel's arm and Grendel retreats, mortally wounded. In the morning, many of the warriors follow the trail of Grendel's blood to a mere. A scop recites a lay about Sigemund and Heremod. Hrothgar praises Beowulf's victory. At a celebratory feast, Beowulf receives generous rewards and a scop narrates a lay.
The night of the feast, Grendel's mother enters the hall, intent on revenge. She carries off Æschere, a favorite thane of Hrothgar. Beowulf is again summoned. With a company of Danes and Geats, Beowulf immediately heads to the mere. He plunges into the water, reaches the bottom and is dragged by Grendel's mother to her cavern, where she has the upper-hand in the combat. Beowulf suddenly discovers a curious and very large sword and slays his foe. He cuts off Grendel's head and swims to the surface, carrying also the hilt of the sword. Back at Heorot, Beowulf recounts his experiences, to which Hrothgar responds with moralising advice. In the morning, Beowulf sails home, where he again gives an account of events. He shares the spoils with his uncle and lives in Geatland with great honour.
Many years later, after Beowulf has ruled the Geats for fifty years, the hoard of a dragon is robbed by a fugitive slave and the dragon responds by laying waste to surrounding lands. Beowulf resolves to neutralise the threat alone. Before departing, he reviews his life. The dragon quickly overwhelms Beowulf with flame, and all his companions flee with the exception of Wiglaf, who is mindful of his obligations of loyalty and gratitude. Together they kill the dragon, but Beowulf has received a mortal wound. The dying hero tells Wiglaf to bring some of the treasure out of the hoard and requests that a mound be built for him on the headland. Wiglaf sends news home of Beowulf's death and the messenger foretells the disaster that will ensue. The Geat warriors return to the hoard, push the dragon's body into the sea and bear the king's body to the headland. A pyre is built on the headland, and the dragon's hoard is buried under a funeral mound. Twelve warriors ride around the mound, praising Beowulf's deeds and virtues.