Set in Denmark in the 6th century, the film opens with King Hrothgar celebrating the construction of his mead hall, Heorot. However, the noise disturbs and enrages the monstrous creature Grendel, who attacks the mead hall and slaughters nearly all of the people there. He retreats to his mother's cave when Hrothgar challenges him to fight. His mother, a water-demon, takes interest in Hrothgar's presence. Hrothgar has the mead hall sealed and offers half of the gold in his kingdom for a hero to kill Grendel. Beowulf and his group of Geats arrive by ship, and Beowulf agrees to slay Grendel. The Geats bed down in the mead hall when challenged by Unferth, Hrothgar's skeptical advisor. Their singing attracts Grendel, and Beowulf mortally wounds him in unarmed combat by breaking his left arm off. Grendel crawls back to his cave and later dies under his mother's caresses.


Grendel's vengeful mother kills all the rest of the Geats except Beowulf and his friend Wiglaf. Hrothgar reveals that Grendel's mother killed the men, and Beowulf decides to kill her too when Hrothgar says that she is the last of demonkind. Hrothgar does not reveal where Grendel's father is but offers the curious opinion that he poses no threat to them.

Beowulf and Wiglaf go to the cave where Grendel's mother lives, but only Beowulf ventures in, armed with the sword Hrunting and a golden horn shaped like a dragon. Grendel's mother appears as a golden naked woman and seduces Beowulf into giving her a son who will become legendary in later years. Beowulf returns to Heorot with Grendel's severed head which is unceremoniously tossed into the ocean. He then dissembles about killing Grendel's mother and losing Hrunting and the horn to her. When Hrothgar privately confronts Beowulf he lets slip with the truth. Hrothgar then reveals that he was Grendel's father. Thus freed from the curse of Grendel's mother Hrothgar implies that the burden is now Beowulf's to bear. He informs his people that Beowulf will become king upon his retirement and promptly plunges to his death.

Years later an aging Beowulf shares his bed with a young mistress named Ursula while queen Wealtheow suffers quietly. A dragon suddenly appears and burns down a nearby village, leaving Unferth as the only survivor and carrying the message of "the sins of the fathers". The horn Beowulf gave to Grendel's mother is also found by Unferth's servant Cain. Beowulf realizes that the dragon is in fact his own son and ventures forth to kill both him and his demon mother. The dragon attacks Heorot and attempts to kill Wealtheow and Ursula. Beowulf stabs the dragon in the chest, draining its ability to breathe fire, and then he literally rips its heart out. Beowulf and the dragon fall from a great height onto the beach far below. The dragon's form dissolves away into a hairless, golden, physically perfect man resembling Beowulf before his body washes away. Beowulf tells Wiglaf the truth before dying.

Beowulf is given a Norse funeral but Wiglaf, now king, finds the horn and sees Grendel's mother embrace Beowulf's body and disappear beneath the ocean before rising up again with an expectant smile on her face and Wiglaf standing in the water looking at her.

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