The first act shows the hall of the castle of the Gibichungs near the Rhine.  Here dwell Gunther and his sister Gutrune, and their half-brother Hagen, whose father was the Nibelung Alberich.  Hagen knows the story of the ring, and that its present possessor is Siegfried, and he devises a crafty scheme for getting Siegfried into his power.  Gunther is still unmarried, and, fired by Hagen’s tale of the sleeping Valkyrie upon the rock of fire, yearns to have Bruennhilde for his wife.  Hagen therefore proposes that Gutrune should be given to Siegfried, and that the latter, who is the only hero capable of passing through the fire, should in return win Bruennhilde for Gunther.  In the nick of time Siegfried arrives.  Hagen brews him a magic potion, by virtue of which he forgets all his former life, and his previous love for Bruennhilde is swallowed up in a burning passion for Gutrune.  He quickly agrees to Hagen’s proposal, and assuming the form of Gunther by means of the Tarnhelm, he departs once more for Bruennhilde’s rock.  Meanwhile Bruennhilde sits at the entrance to her cave upon the fire-girt cliff, musing upon Siegfried’s ring.  Suddenly she hears the old well-known Valkyrie war-cry echoing down from the clouds.  It is her sister Waltraute, who comes to tell her of the gloom that reigns in Valhalla, and to entreat her to give up the ring once more to the Rhine-maidens, that the curse may be removed and that the gods may not perish.  Bruennhilde, however, treasures the symbol of Siegfried’s love more than the glory of heaven, and refuses to give it up.  She defies the gods, and Waltraute takes her way sadly back to Valhalla.  Now Siegfried’s horn sounds in the distance far below.  Bruennhilde hurries to meet him, and is horrified to see, not her beloved hero, but a stranger appear upon the edge of the rocky platform.  The disguised Siegfried announces himself as Gunther, and after a struggle overcomes Bruennhilde’s resistance and robs her of the ring.  This reduces her to submission; he bids her enter her chamber and follows her, first drawing his sword, which is to lie between them, a proof of his fidelity to his friend.

The second act begins with the appearance of Alberich, who comes to incite his son Hagen to further efforts to regain the ring.  Siegfried appears, and announces the speedy arrival of Gunther and Bruennhilde.  Hagen thereupon collects the vassals, and tells them the news of their lord’s approaching marriage, which is received with unbounded delight.  Bruennhilde’s horror and amazement at finding Siegfried in the hall of the Gibichungs, wedded to Gutrune and with the ring so lately torn from her upon his finger, are profound.  She accuses him of treachery, declaring that she is his real wife.  Siegfried, for whom the past is a blank, protests his innocence, declaring that he has dealt righteously with Gunther and not laid hands upon his wife.  Bruennhilde, however, convinces Gunther of Siegfried’s deceit, and together with Hagen they agree upon his destruction.