The narrator tells us that a clan called the Spear-Danes, in "days gone by" (that's the past, to you) had some awesome heroic kings.
The first of these hero-kings is Shield Sheafson, who is basically awesome because he could rampage and pillage with the best of them—both on the battlefield and in the mead hall, if you get our drift. He is an orphan, but he eventually becomes king and then subjugates other nearby clans, making them pay tribute to the Spear-Danes.
Shield's son is Beow, a wise, prudent, valiant prince who sympathizes with the hardships his people have endured.
Shield dies in the prime of his life and is buried at sea in a ship loaded with wealth and treasures, according to the custom of the Spear-Danes. It sails off and nobody knows what happens to it.
Beow becomes king and rules long and well. He is succeeded by Halfdane, a warlord who has three sons, Heorogar, Hrothgar, and Halga, and one daughter. Halfdane's daughter isn't given a name in the poem, although we assume that she had one, but we do learn that she marries Onela, the king of the Swedes.
Halfdane's son Hrothgar is fortunate in battle and gradually amasses the most followers and wealth of any of the princes, so he becomes king after his father.
To consolidate his power, Hrothgar builds a grand mead-hall, Heorot Hall, which does dual duty as a throne room and a hangout for the powerful members of his "court." Okay, we say court, but it's really just a bunch of tough barbarians in grimy, blood-smeared armor sitting around a rough wooden table drinking mead and talking about battles.
When Heorot Hall is finished, Hrothgar gives out treasures to his followers to celebrate and thank them for their help. The narrator knows, however, that the hall is doomed to burn down in the midst of a bloody battle. So much for suspense, right? But don't worry; it's still going to be totally awesome.
A local demon named Grendel is disturbed by the presence of Heorot Hall; like your neighbors, he hates to hear everyone drinking and partying and listening to music. It's even worse because the bard is singing about God's creation of the world, which is something that drives demons crazy.
Grendel, who is one of the monstrous descendants of the Biblical outcast Cain, has been hanging around the marshes in the area for a long time—like, since Cain.
One night, Grendel attacks Heorot Hall in the dead of night, when everyone is in a sleepy, alcohol-induced stupor. Grendel kills thirty men and takes their corpses back to his lair. Now that guy knows how to throw a party.
In the morning, everyone is shocked and horrified by the destruction that Grendel caused. King Hrothgar is humiliated and seems helpless.
The next night, Grendel comes back and marauds some more. In fact, Grendel shows up almost every night, hunting down Danes and murdering them.
Eventually, Heorot Hall is abandoned; everyone has been killed or fled. For twelve years, Grendel rules the hall at night.
The story of Grendel's rampages, the suffering of the Spear-Danes, and the helplessness of King Hrothgar spreads throughout the world. People tell stories and write sad songs about it. Nothing can stop Grendel; he won't negotiate or even accept a ransom or bribe, and he kills everyone, young and old alike.
(This is especially disturbing for a medieval European audience, because paying money to end feuds and wars was part of their code of behavior. Refusing to do so seems insane, because otherwise how would the killing ever stop?)
So Grendel pretty much takes over Heorot Hall, although God keeps him from approaching Hrothgar's throne. (That kind of thing happens when you're the descendant of Cain, apparently.)
Everyone wants to give King Hrothgar their two cents about how to get rid of Grendel. Some of them give advice about military strategy; others turn to idolatry and offer sacrifices to pagan gods.
Despite all the good advice, Hrothgar and his followers can't defeat Grendel, and he keeps killing the Danes in the darkness of night.
Across the water from the Danes in Geatland (today part of Sweden), the mightiest warrior on earth, a follower of King Hygelac, decides that he will travel to the Spear-Danes and help King Hrothgar defeat Grendel. He orders that a boat be made ready.
Everyone knows better than to argue with this warrior or try to stop him. Instead, they help him get ready. They enlist fourteen other warriors to accompany him.
Loading their ship with weapons, the mysterious hero and his followers set sail. After a day at sea, they come to the Danish coast, where they thank God for their easy passage.
The Danish warrior on lookout duty rides down to the shore to find out who the new warriors are and whether they're on a mission of peace or of war. He issues a formal challenge to the Geats to explain who they are and why they've come, and he notices how noble and mighty the mysterious hero looks.
The warrior explains that he is a Geat, a follower of King Hygelac, and the son of a noble and famous warlord named Ecgtheow. He explains that the Geats haven't come to fight the Danes, but to help fight the demonic enemy of the Danes.
The lookout believes the warrior and allows him to pass; he even promises that he'll set a guard over the Geats' boat to keep it safe until they need to return home.