The poem begins with a brief genealogy of the Danes. Scyld Shefing was the first great king of the Danes, known for his ability to conquer enemies. Scyld becomes the great-grandfather of Hrothgar, the king of the Danes during the events of Beowulf. Hrothgar, like his ancestors before him, is a good king, and he wishes to celebrate his reign by building a grand hall called Heorot. Once the hall is finished, Hrothgar holds a large feast. The revelry attracts the attentions of the monster Grendel, who decides to attack during the night. In the morning, Hrothgar and his thanes discover the bloodshed and mourn the lost warriors. This begins Grendel's assault upon the Danes.
Twelve years pass. Eventually the news of Grendel's aggression on the Danes reaches the Geats, another tribe. A Geat thane, Beowulf, decides to help the Danes; he sails to the land of the Danes with his best warriors. Upon their arrival, Hrothgar's thane Wulfgar judges the Geats worthy enough to speak with Hrothgar. Hrothgar remembers when he helped Beowulf's father Ecgtheow settle a feud; thus, he welcomes Beowulf's help gladly.
Heorot is filled once again for a large feast in honor of Beowulf. During the feast, a thane named Unferth tries to get into a boasting match with Beowulf by accusing him of losing a swimming contest. Beowulf tells the story of his heroic victory in the contest, and the company celebrates his courage. During the height of the celebration, the Danish queen Wealhtheow comes forth, bearing the mead-cup. She presents it first to Hrothgar, then to the rest of the hall, and finally to Beowulf. As he receives the cup, Beowulf tells Wealhtheow that he will kill Grendel or be killed in Heorot. This simple declaration moves Wealhtheow and the Danes, and the revelry continues. Finally, everyone retires. Before he leaves, Hrothgar promises to give Beowulf everything if he can defeat Grendel. Beowulf says that he will leave God to judge the outcome. He and his thanes sleep in the hall as they wait for Grendel.
Eventually Grendel arrives at Heorot as usual, hungry for flesh. Beowulf watches carefully as Grendel eats one of his men. When Grendel reaches for Beowulf, Beowulf grabs Grendel's arm and doesn't let go. Grendel writhes about in pain as Beowulf grips him. He thrashes about, causing the hall to nearly collapse. Soon Grendel tears away, leaving his arm in Beowulf's grasp. He slinks back to his lair in the moors and dies.
The Danes, meanwhile, consider Beowulf as the greatest hero in Danish history. Hrothgar's minstrel sings songs of Beowulf and other great characters of the past, including Sigemund (who slew a dragon) and Heremod (who ruled his kingdom unwisely and was punished). In Heorot, Grendel's arm is nailed to the wall as a trophy. Hrothgar says that Beowulf will never lack for riches, and Beowulf graciously thanks him. The horses and men of the Geats are all richly adorned, in keeping with Hrothgar's wishes.
Another party is held to celebrate Beowulf's victory. Hrothgar's minstrel tells another story at the feast, the story of the Frisian slaughter. An ancient Danish king had a daughter named Hildeburh; he married her to a king of the Frisians. While Hnaef, Hildeburh's brother, visited his sister, the Frisians attacked the Danes, killing Hnaef and Hildeburh's son in the process. Hengest, the next leader of the Danes, desired vengeance, and in the spring, the Danes attacked the Frisians, killing their leader and taking Hildeburh back to Denmark.
After this story is told, Wealhtheow presents a necklace to Hrothgar while pleading with her brother-in-law Hrothulf to help her two young sons if they should ever need it. Next she presents many golden treasures to Beowulf, such as necklaces, cups, and rings. Soon the feast ends, and everyone sleeps peacefully.
In the night, Grendel's mother approaches the hall, wanting vengeance for her son. The warriors prepared for battle, leaving enough time for Grendel's mother to grab one of Hrothgar's counselors and run away. When Beowulf is summoned to the hall, he finds Hrothgar in mourning for his friend Aeschere. Hrothgar tells Beowulf where the creatures like Grendel livein a shadowy, fearful land within the moors.
Beowulf persuades Hrothgar to ride with him to the moors. When they reach the edge of the moors, Beowulf calls for his armor, takes a sword from Unferth, and dives into the lake. After a long time, Beowulf reaches the bottom of the lake, where Grendel's mother is waiting to attack. Beowulf swings his sword, but discovers that it cannot cut her, so he tosses it away. They then wrestle until Beowulf spies a large sword nearby. He grabs it by the hilt and swingskilling Grendel's mother by slicing off her head. Still in a rage, Beowulf finds the dead Grendel in the lair and cuts off his head as a trophy.
As they wait, the Danes have given up all hope for Beowulf because he has been underwater for such a long time. They are shocked when Beowulf returns with Grendel's head and the hilt of the sword (which melted with the heat of Grendel's blood). They bear the hero and his booty back to Heorot, where another celebration takes place. Beowulf recounts his battle; Hrothgar praises him and gives him advice on being a king. A grand feast follows, and Beowulf is given more priceless treasures. The next morning, the Geats look forward to leaving Denmark. Before they leave, Beowulf promises aid for Hrothgar from the Danes. Hrothgar praises Beowulf and promises that their lands will have an alliance forever. As the Geats leave, Hrothgar finds himself wishing Beowulf would never leave.
The Geats return with much rejoicing to their homeland, where their king Hygelac and his queen Hygd greet them. In an aside, the narrator compares Hygd to the queen of the ancient Offa, who is not tamed until Offa comes to subjugate her. Beowulf tells his lord the events of his trip to Denmark. In the process, he tells another story that had previously been unmentioned. Hrothgar betrothed his daughter Freawaru to a prince of the Heathobards in order to settle an old feud. Beowulf speculates that someone will goad this Heathobard prince to take vengeance upon the Danes for all their past wrongs. Hygelac praises Beowulf for his bravery and gives him half the kingdom. They rule the kingdom together in peace and prosperity. Hygelac is killed in a battle soon after, so Beowulf becomes king of the Geats and rules the kingdom well.
In the fiftieth year of Beowulf's reign, a monster arises to terrorize the Geats. A treasure trove was left by an ancient civilization, which guarded it jealously until only one member of the race was left. After the last person's death, a fire-breathing dragon found the treasure and guarded it for three hundred years. One day, a slave stumbled upon the treasure and stole a cup as an offering to his lord. The dragon awakened to find something missing from his treasure, and began his rampage upon the Geats.
One day, Beowulf learns that this dragon has destroyed his own great hall. This attack sends him into deep thought. Soon he orders a shield to use for battle, but not without a heavy heart at what may happen to him. He recalls Hygelac's death in battle and his own narrow escape from this battle. He recalls a number of battles he has seen as he travels to the dragon's lair with eleven of his thanes. The servant who stole the cup leads them to the lair.
As they wait to attack the dragon, Beowulf recounts the Geat royal family's plight, in which Hygelac's oldest brothers killed each other and left their father to die of a broken heart. Beowulf says he served Hygelac well, and a sword (named Naegling) that he won while serving Hygelac will help him save the kingdom once again. Beowulf leads the charge to the dragon's cave. The shield protects him from the dragon's flames, but his men flee in fear, leaving only one man behind. This man is Wiglaf, Beowulf's kinsman through Ecgtheow. Wiglaf becomes angry, but swears that he will stay by Beowulf's side.
Just then the dragon rushes up to them. Beowulf and the dragon swing at each other three times, finally landing mortal blows upon each other the last time. The dragon is beheaded, but Beowulf is bitten and has a mortal poison from the dragon flowing through his body as a result. Wiglaf bathes his lord's body as Beowulf speaks on the treasure. He says that Wiglaf should inherit it as his kinsman; then he dies.
After his death, the cowards return, to be severely chastised by Wiglaf. He sends a messenger to tell the people of their king's death. The messenger envisions the joy of the Geats' enemies upon hearing of the death of Beowulf. He also says that no man shall ever have the treasure for which Beowulf fought. Wiglaf and Beowulf's thanes toss the dragon's body into the sea. They place the treasure inside a mound with Beowulf's body and mourn for "the ablest of all world-kings."