Heorot Hall, built by Hrothgar, finds itself to be attacked by Grendel—a beast that is tortured by the sounds of people celebrating from the hall. After twelve long years of constant attacks from Grendel every night, Hrothgar calls for Beowulf's assistance in slaying the beast.

When they arrive, Beowulf plans not to use any weapons when he battles Grendel—as he sees it unjust that he fight Grendel with a blade, as the monster does not wield one himself. Later that night, Beowulf and his men stay in Heorot Hall and await for Grendel's arrival.

Grendel soon arrives and immediately feasts on one of Beowulf's men. Meanwhile, Beowulf—who had been acting asleep—sudden leaps and grabs the beast's hand to clench it tight. Grendel's own claws dig at his skin, peircing his own flesh, the pain becoming unbearable. The two battle each other violently, the wailing and writhing of the beast awakening Beowulf's soldiers—though their blades do no harm to Grendel's skin.

Soon, Beowulf is able to tear away Grendel's arm, all the way to his shoulder. Fatally injured, Grendel retreats back to his home, where he dies. Beowulf later displays Grendel's arm for all to see at Heorot.

The next night, after celebrating Grendel's defeat, Hrothgar and his men sleep in Heorot Hall. However, Grendel's mother, Furious that her son had been killed, seeks out revenge. Beowulf had been absent from the Hall when Grendel's mother had attacked, allowing her to slay one of Hrothgar's most loyal advisors and behead him.

Later that day, Beowulf, Hrothgar, and their men go and hunt for Grendel's mother, and soon find her lair under a lake. Before his descent, Beowulf is offered a sword by Unferth—a warrior that had challenged Beowulf's ability. When he reaches the bottom, the mighty warrior finds himself dragged into the lair of Grendel's mother, where the two engage in combat.

At first, Grendel's mother overpowers Beowulf, the Hrunting unable to harm her. She throws Beowulf into the ground, and tries to kill him with a sword—though his armor saves him. The warrior spots another sword stuck on the wall, apparently able to kill giants. He beheads Grendel's mother with it, before venturing further into the cave, where he finds Grendel's body and beheads it as well. However, the sword melts from the monster's hot blood, leaving only the hilt. Carrying the head of Grendel and the hilt of the sword, Beowulf ascends back up to the surface, where he presents his findings to Hrothgar back in Heorot and is rewarded for his service.

When Beowulf returns home, he becomes king of his own people. Several decades later, a frenzied dragon attacks, burning everything in sight. Beowulf steps up and battles the dragon —soon seeing himself outmatched. Upon seeing this, his men retreat, but one brave warrior stays behind and helps Beowulf defeat the dragon—Wiglaf. However, when they slay the dragon, Beowulf is fatally wounded.

After Beowulf dies and his men return, Wiglaf harshly reprimands the other soldiers, blaming their cowardice for the death of their king. Beowulf is cremated as his people mourn him, a barrow built in his memory