Brünnhilde sends Siegfried off to explore the world. Siegfried gives her the ring that he took from Fafner’s cave as a token of his faithfulness.
At the Gibichung castle, he encounters Gunther, Gunther’s sister Gutrune, and their half-brother Hagen (whose father is Alberich). Urged on by Hagen, Gunther and Gutrune give Siegfried a drink that makes him forget Brünnhilde and fall in love with Gutrune. Once the potion has worked, Gunther tells Siegfried about the goddess enclosed by flames and asks for Siegfried’s help in winning her. In return, Gunther pledges to allow Siegfried to marry Gutrune. No longer remembering Brünnhilde, Siegfried agrees. Brünnhilde, back at her mountain home, refuses the plea of her sister Waltraute to relinquish the ring given to her by Siegfried. Disguised as Gunther, Siegfried comes through the flames, takes the ring from Brünnhilde, and claims her as Gunther’s bride.
When the true Gunther takes her back to the Gibichungs and she sees Siegfried, Brünnhilde’s rage knows no bounds. Siegfried swears on Hagen’s spear that he does not know her, and Brünnhilde swears that Hagen’s spear will avenge her. She and Hagen convince Gunther to plan Siegfried’s death.
The next day, at a hunt organized by Gunther, Siegfried tells his life story to the other men. Hagen gives him a drink that restores his memory. When Siegfried sings of Brünnhilde, Hagen stabs him in the back with his spear. The hunting party returns and Brünnhilde sees Siegfried’s body, whereupon she resolves to return the ring to the Rhine and end its curse. She takes the ring from Siegfried’s hand and plunges into his funeral pyre. The funeral flames spread to burn Valhalla and destroy the gods. The ring is restored to the Rhine, and Brünnhilde’s sacrifice begins the world afresh.