The epic poem opens with the praise of the early Danish kings, tracing the lineage of Scyld and leading down to King Hrothgar, who commands the construction of the magnificent hall Heorot. Soon, a malignant swamp-fiend named Grendel, bearing the curse of Cain, grows jealous of the festive joy within and begins terrorizing the Danes by nightly devouring thirty men. Hearing of these calamities across the sea, Beowulf, a famed thane of the Geats, sails with fourteen warriors to aid Hrothgar. After a feast where Beowulf silences the envious taunts of Hunferth by recounting his swimming contests, night falls and Beowulf takes charge of the hall. Grendel breaks in, devours a warrior, but is caught in a fatal grip by Beowulf, who rips the monster's arm from its shoulder and forces him to flee mortally wounded to the fens.
The following night, Grendel's mother arrives to avenge her son, carrying off Hrothgar's trusted councillor Æschere. Beowulf and his men track her to the grim, fire-lit mere, where he plunges into the dark waters and slays the water-demon with an ancient giant-sword, bringing back her head and Grendel's head to Hrothgar. Laden with rich gifts and glowing praise, Beowulf sails back to Geatland and recounts his glorious adventures to King Hygelac. After the deaths of Hygelac and his son Heardred, Beowulf ascends the throne and rules the Geats prosperously for fifty years.
Peace is shattered when a dragon, brooding over a hidden hoard of ancient gold, begins ravaging the country with fire after a runaway thief steals a single cup. Beowulf, now aged and sensing his impending end, sets out with twelve men and his faithful kinsman Wiglaf to confront the beast. In the fierce battle, Beowulf's sword snaps, and his terrified companions flee into the woods, save for Wiglaf, who rushes to his lord's side. Together, they slay the dragon, but not before Beowulf sustains a fatal venomous wound. Before he dies, Beowulf gazes upon the recovered treasure and issues final commands for his burial, asking that a bright barrow be raised on a headland for passing sailors. The poem concludes with a solemn funeral and mournful dirges, accompanied by dark prophecies of future wars and tribal ruin.