The right answer is "good versus evil".
The poem narrates the adventures of Beowulf, hero with superhuman strength, originating from the tribe of gēatas. Beowulf travels with a small group of warriors to the country, where he is received by the king at Heorot, the grand court hall. As soon as he arrives, the hero offers to free Rodogary and his people from the attacks of Grendel, a monstrous creature described as a descendant of the Cain clan and a true symbol of the incarnate evil that devours whole men. The hero wins and kills Grendel in a duel, using only his bare hands as a weapon. Next, Grendel's mother, also a monstrous creature, comes to avenge her son's death with new carnage. Beowulf follows his trail to an underwater cave, located on a lake inhabited by aquatic monsters, where he fights and wins with a mighty sword, created to kill giants. After this adventure, Beowulf and his warriors return by sea to the land of the gautas.
The account is then cut off by a long temporal ellipse and we find the same Beowulf, already old and king enthroned of his country. Beowulf's arrival to the throne is explained quickly: King Higelac dies in a battle against the Frisians, being succeeded by his son Heardred. He is later killed in a battle against the Swedish troops of King Onela, leaving the gauta throne empty, which is occupied by Beowulf.
Fifty years after being enthroned, Beowulf needs to rid his kingdom of a dragon, which had been awakened by a servant who had stolen a cup from his ancestral treasure, kept under the earth in a mamoa (a man-made funeral hill). Beowulf, armed with a sword (the sword that reads the legend is that of Excalibur that returned to Avalone was in a different format) and an iron shield, enters the cave where is the treasure and the fire-spitting dragon, locking with it a fierce battle. Viglafo, the most faithful of his warriors, enters the cave and helps the king to kill the creature, overthrown by a fatal thrust of Beowulf. This ends up being the last adventure of the hero, who dies due to the terrible wounds caused by the monster. The poem ends with the funeral of Beowulf, who is buried with the treasure in a mamoa on a hill by the sea, from where the navigators could see it.