The poem begins with a genealogy of the Danish royal family. Scyld Shefing, the founder of the dynasty, becomes King of the Danes not through wealth (for he comes from an impoverished family) but through his ability to sack the enemies. He has a son named Beow (called Beowulf), also called a great king because he gave his treasures to his men "to make sure that later in life his beloved companions will stand by him." Upon Scyld's death, the people bury him and his treasures at sea in a traditional Germanic ceremony. Beow comes to the throne, and has a son, Healfdene. Healfdene, in turn, becomes the father of Hrothgar, the King of the Danes at the beginning of the story.
Like his ancestors, Hrothgar has kept the kingdom prosperous through winning battles and honoring his warriors. He decides to build a lavish hall named Heorot. Soon it is finished, and it becomes a great hall of feasting until the demon Grendel hears the happiness in the hall and wishes to destroy it. Thus Grendel begins the bloody, 12-year rampage on Heorot that leaves Hrothgar and his people powerless to stop him.
The prologue recounts an age of glory for the Danes, yet it has a bitter tone. The "grand old days" of heroes has been replaced with an era of cowardice. From his description, we see that Scyld is a mighty king who can defeat anything. Compare this to his great- great- grandson Hrothgar, who is only fighting one enemy, yet allows the enemy to take over his kingdom completely without attempting to kill the monster himself. The narrator also foreshadows another weakness in the later Germanics. Beowulf of the Danes keeps his men faithful by paying them treasures; later in the poem, even treasure will not keep Beowulf of the Geats' men from leaving him to fight alone.
Heorot is Old English for "the hart," and indeed the splendor of the hall flees as a deer. The hall and the arrival of Grendel are likened to the story of the Creation and the Flood: a paradise is built, and the people enjoy its fruits until they are cursed with a disaster (even a family member of Cain is involved). Despite their knowledge of God and Christian ritual, the people turn to the pagan rituals: the Danes still expect the pagan gods to help them from the dire situation, and Grendel cannot be "bought off" with the traditional Danegeld, paid to an enemy to stop his attack.