The poem opens in the kingdom of the Danes, where King Hrothgar has constructed a magnificent mead-hall named Heorot. This hall soon becomes the target of Grendel, a ferocious demon descended from Cain, who terrorizes the Danes by night for twelve years. Hearing of this devastation, the valiant Geatish hero Beowulf sails across the sea with a band of chosen companions to offer his assistance.
Upon arriving, Beowulf confronts Grendel in a brutal bare-handed struggle inside Heorot, successfully tearing the monster's arm from its socket and sending him away to die. The Danes celebrate this deliverance with immense gratitude and lavish gifts, believing the terror has ended. However, Grendel's equally monstrous mother arrives the following night to avenge her son, dragging away one of Hrothgar's closest advisors and plunging the hall back into sorrow.
Undeterred, Beowulf tracks the female demon to her lair beneath a murky lake. After a desperate underwater battle against overwhelming odds, he slays her and returns to the surface bearing Grendel's head and the hilt of a magical sword. Hrothgar showers him with further treasures and wisdom, warning him against the perils of pride before the hero returns triumphantly to his own land.
Decades later, Beowulf has become the aging king of the Geats, having ruled his people wisely for fifty years. Peace is shattered when a vicious, fire-breathing dragon begins ravaging the countryside after a thief steals a cup from its hidden hoard. Though weighed down by age, Beowulf refuses to ignore the threat and marches out to face the beast with a small retinue.
During the fierce encounter, Beowulf is deserted by most of his panic-stricken warriors, save for his loyal kinsman Wiglaf. Together, the old king and the faithful youth manage to slay the dragon, but not before Beowulf sustains a fatal wound from its venomous bite. The poem concludes with a profound lament for the fallen leader, whose body is burned on a magnificent funeral pyre while his people mourn their greatest protector.