The Danes celebrate the completion of Heorot with a feast, at which Hrothgar's bard sings about the creation of the Earth.
As a mead-hall, Heorot, like the Earth, is a source of life for its people.
But the monster Grendel, a descendent of Cain and therefore an outcast from society, hears the singing from his home in the swamp. These sounds of community and merriment fill him with anger.
Grendel, a monster with no tribe or people, hates any reminder that others do have these necessities.
That night, Grendel visits Heorot as the Danes are sleeping. Grendel seizes thirty warriors and carries them to his den to kill and eat them. The next night Grendel repeats his raid. After that, the Danes abandon Heorot to Grendel after nightfall. This situation continues for twelve years.
Grendel turns Heorot, the heart of Danish society, into a slaughterhouse. Grendel the outcast has, symbolically, made the Danes outcasts. The defeat shames the Danes.
The narrator notes that unlike men, Grendel has no desire to end the feud, or to pay compensation for those he kills and thus make peace with their families. Hrothgar can neither make peace with Grendel, nor destroy him.
Because Grendel is a being outside of society, there is no way for Danes to deal with him.
In despair, Hrothgar and the Danes prayed to their heathen gods. The narrator pities them, since they did not know of the true "Lord God" and were therefore praying to demons to save them from a monster.
Here the narrator says that the Danes of the time were not Christians. However, the characters speeches later in the text seem to contradict this statement.