What is the plot of story Nibelungenlied base on?

The plot of the Nibelungenlied is based on the Germanic heroic legend of Siegfried the Dragon-slayer. The story begins with Siegfried's arrival at the court of King Gunther of Burgundy. Siegfried helps Gunther win the hand of the beautiful Kriemhild, Gunther's sister, by defeating the dragon Fafnir and acquiring the treasure of the Nibelungs.

After the wedding, Siegfried is betrayed by Hagen, Gunther's chief advisor, who murders him in the forest. Kriemhild swears revenge and eventually marries King Etzel of Hungary. Years later, she invites the Burgundians to a feast at Etzel's court, where she has them massacred in revenge for Siegfried's death. In the end, Kriemhild is herself killed by Hildebrand, Dietrich von Bern's right-hand man.