Beowulf replies that he will immediately set forth on this new task. He says he will not allow the monster to escape, and he encourages Hrothgar not to lose heart. They saddle the horses and Hrothgar, Beowulf and some of his men go off in pursuit, following the monster's tracks in the forest paths, across the moors and on difficult terrain. At the foot of a cliff, near the monster's sea-den, they find the head of Aeschere.
The water of the mere is full of reptiles, and sea-dragons and other monsters slouch on the slopes of the cliff. Beowulf and his men attack and kill many of them.
Beowulf arms himself for the underwater fight. Unferth, who is not courageous enough to fight the monster himself, gives Beowulf his rare and ancient sword named Hrunting. Beowulf speaks to Hrothgar, asking him to take care of his men should he, Beowulf, not survive the battle. He also asks that the gifts Hrothgar bestowed on him should be sent to his king, Hygelac, and that the sword he is about to use should be returned to Unferth.
With that, Beowulf dives into the lake. It takes him nearly a day to reach the bottom. From her lair, the monster senses the presence of a human. She grips Beowulf hard, but his armor saves him from injury. But she drags him to her lair. He is attacked by sea beasts.
When they reach her lair, Beowulf manages to swing his sword at her, it lands on her head. But it fails to do any damage. Without losing heart, he flings his sword away. He grips the monster and throws her to the floor. She gets up and grips him again, and as they grapple, Beowulf stumbles and falls. Grendel's mother pounces on him with a knife. But again Beowulf's armor saves him, deflecting the blade.
Beowulf manages to get to his feet again, and he grabs a huge sword from her armoury. He swings it and it cuts into her neck, severing the bone. The monster topples to the floor; Beowulf's sword drips blood. Beowulf then uses it to cut off the head of the monster's corpse.
Above the lake, the watching warriors see the water fill with blood, and they assume that Beowulf has been killed. Hrothgar and his men go home, but the fourteen Geat warriors stay on, hoping against hope that Beowulf has survived.
Beowulf returns to the surface carrying the hilt of the sword and the monster's head. His men rejoice to see him.
The first lines of this section reveal much about the social codes of the heroic society. Beowulf says, "It is always better / to avenge dear ones than to indulge in mourning" (lines 1384-85). Avenging a death is the best way for a hero to win glory. It is well to remember that although there are Christian elements in the poem, they all refer to the Old Testament rather than the New. The forgiveness of sins as taught by Christ is absent. The law of the heroic code is an eye for an eye.
The lengthy descriptions of the formidable armor are familiar from earlier passages. Weapons are considered so important they are even given names and their owners boast of their history. Unferth's "rare and ancient" sword, for example, is named Hrunting. Readers familiar with Homer's Iliad will recall similar attitudes to weaponry expressed in that epic poem.
Beowulf's fight with Grendel's mother is a much tougher battle than his earlier fight with Grendel. This seems appropriate. It is as if Beowulf is now having to track evil to its source, in the murky depths of water. The fairy-tale elements are strong here-the hero is able to hold his breath under water for nearly a day, which is how long it takes him to reach the bottom of the lake. But the poet has not forgotten his Christianity either. Beowulf wins because God gives him victory.