Beowulf rules the Geats wisely for fifty years. He inherits the throne after Hygelac, and later his son, Heardred, fall in battle. (Heardred's story is told later.)
A dragon that guards buried treasure is angered when a slave, fleeing from his master, accidentally stumbles on his lair. The dragon was asleep, and the slave stole a goblet. He had not intended to steal, but was so shocked when he saw the dragon he panicked and ran off with the goblet.
An old warrior had buried the treasure a long time ago. All his companions had been killed in war, and he had nothing left to live for.
The dragon, who was driven to search out and guard such hoards, discovered the buried treasure. He has been protecting it for three hundred years and is furious when he discovers the theft. He hunts without success for the thief, and plots his revenge. At night, he wreaks havoc on the people far and wide, leaving nothing alive after his attacks. Before daybreak, he returns to his lair.
The dragon also attacks and destroys Beowulf's home. When Beowulf hears the bad news he falls into deep distress. He feels he must have sinned against God to have deserved such misfortune. Plotting his revenge against the dragon, he orders the construction of an all-iron shield. He does not fear the dragon, and refuses to take a large army with him. He has, after all, always triumphed in the past.
In a brief flashback, the poet recalls how, through his prodigious swimming ability, Beowulf escaped from the battle in which Hygelac was killed. Queen Hygd offered him the throne, since she had no faith in the ability of her son, Heardred, to defend the kingdom. But Beowulf turned down the offer, and supported Heardred as the new king. But then raiders from Sweden, led by Onela, arrived. Although Heardred offered them hospitality, they killed him. Beowulf then ascended to the throne. He avenged the death of Heardred by forming an alliance with the Eadgils. In the ensuing military campaign, Beowulf killed Onela.
Mythology intrudes on history again with the coming of this fifty-foot fire-breathing dragon. This is the third and last great challenge for Beowulf, and it reveals the structure of the epic. The poem is about evenly divided between these three episodes involving different kinds of monsters, and they loosely follow the same structure: the ravages of the demon and its awful strength are described, as are the effects on the human communities. Beowulf then comes to the rescue, the fight itself is described at length, as is the aftermath of the struggle.
There is a difference between the two Grendel monsters and the dragon. Grendel and his mother have vaguely human forms, and Grendel at least is pure evil. There is no reason for his attacks. The other two monsters are goaded into action because they have suffered a wrong. Grendel's mother wants to avenge the death of her son, and the dragon, who quietly minded his own business for centuries, goes on the rampage because a thief stole some of the treasure he was guarding.