Everyone continues to drink wine and enjoy themselves, but the narrator knows that something sinister is about to happen.
King Hrothgar retires for the night, and the rest of the warriors bed down to sleep in Heorot Hall for the night. Because Grendel is dead, they think they're completely safe – but at least one of them is already "marked for death" (1241).
Even in their sleep, the Geatish and Danish warriors are ready for battle. They keep their shields at their heads and their armor, helmet, and spear on the bench just above where they're sleeping.
The warriors go to sleep – but Grendel's mother, grieving for her son, is out marauding in the night, ready to revenge herself on her son's killers.
Grendel's mother comes to Heorot Hall, which is filled with sleeping, unsuspecting warriors. She attacks, and she's nearly as powerful as Grendel himself was.
The warriors wake up and grab their swords and shields; they don't have time to put on their helmets or chain-mail shirts.
Grendel's mother panics. Clutching the corpse of one of Hrothgar's retainers, she flees back to the swamp.
Where's our hero while all this is happening, you ask? Beowulf is somewhere else, because after his heroic exploits Hrothgar didn't want to make him sleep on the floor of the hall with the common soldiers.
In Heorot Hall, everything is chaos. Not only did Grendel's mother kill several men from each tribe, Danes and Geats alike – she also stole Beowulf's trophy, the severed arm of Grendel.
When King Hrothgar hears the news, he is grief-stricken by the death of his retainer, who was his dearest companion.
Hrothgar, wondering if God is ever going to have mercy on him, sends for Beowulf, who doesn't know what has happened yet.
Beowulf asks if Hrothgar has rested, and Hrothgar replies that he can never rest as long as the Danes are plagued by yet another demon. He laments the death of his counselor, whose name, we learn, is Aeschere.
Hrothgar realizes that Grendel's mother is bound to avenge the death of her son. That's how things work in this medieval world: every killing brings about another killing, and deaths turn into blood feuds.
There is a legend, Hrothgar tells Beowulf, among the local people, about two huge monstrous demons, one male and one female, who roam the countryside together. He thinks this explains the appearance of the second monster, Grendel's mother.
Although earlier the poet has described Grendel as the descendant of Cain, here he's described as "fatherless," from a mysterious, ghostly, demonic source. Grendel and his mother live on the moors, where streams disappear into the mist.
A few miles away from Heorot Hall, King Hrothgar tells Beowulf, there is a strange, eerie lake. As far as men can tell, it's bottomless, and at night the water burns. Deer who come to its banks would rather stay where they are and be torn apart by pursuing hounds than jump into it.
Hrothgar thinks this lake must be the place where Grendel's mother lives. He puts his faith in Beowulf, asking him to seek the monster if he dares, and offering more treasures as a reward if he succeeds in killing it and coming back alive.
Beowulf comforts Hrothgar with tough love, telling him that everyone dies and that it's better to avenge the deaths of our loved ones than to sit around mourning them. All warriors can do, he reminds Hrothgar, is try to win fame before they're eventually killed.
Beowulf swears to track down Grendel's mother and kill her.
Hrothgar praises God for Beowulf's promise. Hrothgar, Beowulf, and their followers ride out, tracking Grendel's mother across the moors. She's pretty easy to follow, since she's dragging Aeschere's corpse behind her.
Hrothgar, Beowulf, and their retinue follow Grendel's mother up the cliffs, into places where they have to walk single-file along narrow ledges. Suddenly, they come to a place where some trees grow out at an angle – the path ends. They're at a dead end on a ledge above the watery lair of the demon.
Everyone is disturbed to realize that Aeschere's severed head is lying at the foot of the cliff.
The whole group sits down to watch the water of the lake, which is filled with sea-monsters and reptiles thrashing around. Someone blows a war-horn and the monsters go crazy.
One of the Geats shoots one of the sea-monsters with an arrow, crippling it. Other warriors rush up and stab it with spears, dragging it onto the bank and looking at it in amazement.
Beowulf dons his chain-mail armor and a golden helmet to protect himself from the crushing grip of the monsters.
Unferth lends Beowulf an ancient sword called Hrunting, which was tempered in blood and which has never lost a battle.
The narrator speculates that Unferth must not remember the drunken speech criticizing Beowulf that he made earlier. Unferth simply isn't brave enough to take on the monsters in the way Beowulf is willing to do.
Beowulf says goodbye to Hrothgar, reminding him that he promised to take care of the Geat warriors if Beowulf is killed in battle and to send the treasures Beowulf won to the Geat king, Hygelac. Beowulf also announces that, if he is killed, Unferth will get Hrunting back.
Without further ado, Beowulf leaps into the churning lake. It takes most of a day before he can see the bottom. (You'll just have to suspend your disbelief about the fact that he couldn't breathe if that were the case.)