The main protagonist Beowulf, a hero of the Geats, comes to the aid of Hrothgar, the king of the Danes, whose great hall, Heorot, is plagued by the monster Grendel. Beowulf kills Grendel with his bare hands and Grendel's mother with a sword of a giant that he found in her lair.
Later in his life, Beowulf is himself king of the Geats, and finds his realm terrorised by a dragon whose treasure had been stolen from his hoard in a burial mound. He attacks the dragon with the help of his thegns or servants, but they do not succeed. Beowulf decides to follow the dragon into its lair, at Earnanæs, but only his young Swedish relative Wiglaf, whose name means "remnant of valor",[a] dares join him. Beowulf finally slays the dragon, but is mortally wounded. He is buried in a tumulus or burial mound, by the sea.
Beowulf is considered an epic poem in that the main character is a hero who travels great distances to prove his strength at impossible odds against supernatural demons and beasts. The poem also begins in medias res ("into the middle of affairs") or simply, "in the middle of things", which is a characteristic of the epics of antiquity. Although the poem begins with Beowulf's arrival, Grendel's attacks have been an ongoing event. An elaborate history of characters and their lineages is spoken of, as well as their interactions with each other, debts owed and repaid, and deeds of valor. The warriors form a kind of brotherhood called a "comitatus", which seems to have formed an ethical basis for all words, deeds, and actions.
First battle: Grendel
Beowulf begins with the story of King Hrothgar, who constructed the great hall Heorot for his people. In it he, his wife Wealhtheow, and his warriors spend their time singing and celebrating. Grendel, a troll-like monster descended from the biblical Cain, is pained by the noise, attacks the hall, and kills and devours many of Hrothgar's warriors while they sleep. Hrothgar and his people, helpless against Grendel, abandon Heorot.
Beowulf, a young warrior from Geatland, hears of Hrothgar's troubles and with his king's permission leaves his homeland to help Hroðgar.
Beowulf and his men spend the night in Heorot. Beowulf refuses to use any weapon because he holds himself to be the equal of Grendel. During the battle, Beowulf has been feigning sleep and leaps up to clench Grendel's hand. The two battle until it seems as though the hall might collapse. Beowulf's retainers draw their swords and rush to his aid, but their blades cannot pierce Grendel's skin. Finally, Beowulf tears Grendel's arm from his body at the shoulder and Grendel runs to his home in the marshes and slowly dies.
Second battle: Grendel's Mother
The next night, after celebrating Grendel's defeat, Hrothgar and his men sleep in Heorot. Grendel's mother, angered by the punishment of her son, appears and attacks the hall. She kills Hrothgar's most trusted warrior, Æschere, in revenge for Grendel's defeat.
Hrothgar, Beowulf and their men track Grendel's mother to her lair under a lake. Beowulf prepares himself for battle. He is presented with a sword, Hrunting, by Unferth, a warrior who had doubted him and wishes to make amends. After stipulating a number of conditions to Hrothgar in case of his death (including the taking in of his kinsmen and the inheritance by Unferth of Beowulf's estate), Beowulf dives into the lake. He is swiftly detected and attacked by Grendel's mother. However, she is unable to harm Beowulf through his armor and drags him to the bottom of the lake. In a cavern containing Grendel's body and the remains of men that the two have killed, Grendel's mother and Beowulf engage in fierce combat.
At first, Grendel's mother appears to have prevailed. Beowulf, finding that Hrunting cannot harm his foe, discards it in fury. Beowulf is again saved from his opponent's attack by his armour. Beowulf grabs a magical sword from Grendel's mother's treasure and with it beheads her. Traveling further into the lair, Beowulf discovers Grendel's dying body and severs its head. The blade of the magic sword melts like ice when it touches Grendel's toxic blood, until only the hilt is left. Beowulf carries this hilt and the head of Grendel out of the cavern and presents them to Hrothgar upon his return to Heorot. Beowulf then returns to the surface and to his men at the "ninth hour" (about 3 pm). He returns to Heorot, where Hrothgar gives Beowulf many gifts, including (possibly) the sword Nægling, his family's heirloom. The hilt prompts a long reflection by the king, sometimes referred to as "Hrothgar's sermon", in which he urges Beowulf to be wary of pride and to reward his thanes.
Third battle: The Dragon
Beowulf returns home and eventually becomes king of his own people. One day, fifty years after Beowulf's battle with Grendel's mother, a slave steals a golden cup from the lair of an unnamed dragon at Earnaness. When the dragon sees that the cup has been stolen, it leaves its cave in a rage, burning everything in sight. Beowulf and his warriors come to fight the dragon, but Beowulf tells his men that he will fight the dragon alone and that they should wait on the barrow. Beowulf descends to do battle with the dragon but finds himself outmatched. His men, upon seeing this display and fearing for their lives, creep back into the woods. One of his men, however, Wiglaf, who finds great distress in seeing Beowulf's plight, comes to Beowulf's aid. The two slay the dragon, but Beowulf is mortally wounded. After Beowulf's death, he is ritually burned on a great pyre in Geatland while his people wail and mourn him. After, a barrow, visible from the sea, is built on his remains (Beowulf lines 2712–3182).