For twelve years the monstrous Grendel has raided King Hrothgar's hall Heorot in Denmark by night, killing and devouring the king's retainers, until the Geatish warrior Beowulf, hearing of the trouble, crosses the sea with a company of men to offer his help. Beowulf deliberately fights Grendel unarmed, wrestling him in the dark until he tears the monster's arm entirely from its socket; Grendel flees mortally wounded to his lair and dies there, and the arm is hung in Heorot as a trophy.
The following night, Grendel's mother comes to Heorot to avenge her son, killing one of Hrothgar's closest counselors before retreating to her lair beneath a monster-infested lake. Beowulf dives into the lake, fights her in an underwater cave using a giant's sword he finds hanging on the wall there after his own weapons fail against her, and beheads her, along with the already-dead Grendel's corpse, before returning to the surface with both trophies and being richly rewarded by Hrothgar.
The poem frames Grendel as a descendant of the biblical Cain, cursed and driven into the wilderness for his ancestor's crime of fratricide, a detail that folds the Anglo-Saxon poem's pre-Christian heroic material into a Christian moral framework, presenting Grendel's raids as an extension of an ancient exile from God's favor rather than simply an unexplained monster attack.