Mime, Alberich's brother, is forging a blade in his cave within the forest. The Nibelung is plotting to obtain the Ring for himself, having raised Siegfried to kill Fafnir for him. He needs a sword for Siegfried to use, but the youth has broken every blade he has made. Siegfried returns from his wanderings in the forest, demanding to know his parentage, and Mime is forced to explain how he took in Siegfried's mother, Sieglinde, who died giving birth. He shows Siegfried the shards of Nothung, and Siegfried orders him to reforge the sword.
Siegfried departs, leaving Mime in despair: it is beyond his skill to repair Nothung. An old man (Wotan in disguise) abruptly appears at his door. The Wanderer wages his head on answering any three riddles from Mime, and Mime agrees in order to dispose of his unwelcome guest. He asks the Wanderer to name the races that live beneath the ground, on the surface, and the skies. These are the Nibelung, the Giants, and the Gods, as the Wanderer correctly answers. Now Mime is forced to wager his own head on answering the Wanderer's riddles. The Wanderer asks him to name the race most beloved of Wotan, but most harshly treated; the name of the blade that can destroy Fafnir; and the person who can make the blade. Mime gives the answer to the first two riddles: the Wälsungs and Nothung. However, he is unable to answer the last riddle. Wotan spares Mime, telling him that only "he who does not know fear" can reforge Nothung, and leaves Mime's head forfeit to that person.
Siegfried returns and is annoyed by Mime's lack of progress. Mime realizes that the one thing he has not taught Siegfried is fear. Siegfried is eager to learn it, and Mime promises to teach him by bringing him to Fafnir the dragon. Since Mime was unable to forge Nothung, Siegfried decides to do it himself. He succeeds by shredding the metal, melting it, and casting it anew. In the meantime, Mime brews a poisoned drink to offer Siegfried after the youth has defeated the dragon.
The Wanderer arrives at the entrance to Fafnir's cave, where Alberich is keeping a vigil. The old enemies quickly recognize each other. Alberich blusters, boasting of his plans for ruling the World once the Ring is returned to him. Wotan calmly states that he does not intend to obtain the Ring. To Alberich's surprise, Wotan wakes Fafnir and informs the dragon that a hero is coming to fight him. Fafnir dismisses the threat, refuses to surrender the Ring to Alberich, and returns to sleep. Both Wotan and Alberich depart.
At daybreak, Siegfried and Mime arrive. Mime decides to draw back while Siegfried confronts the dragon. As Siegfried waits for the dragon to appear, he notices a woodbird in a tree. Befriending it, he attempts to mimic the bird's song using a reed pipe, but is unsuccessful. He then plays a tune on his horn, which brings Fafnir out of his cave. After a short exchange, they fight, and Siegfried stabs Fafnir in the heart with Nothung.
In his last moments, Fafnir learns Siegfried's name, and tells him to beware of treachery. When Siegfried draws his sword from the corpse, his hands are burned by the dragon's blood, and he instinctively puts them to his mouth. On tasting the blood, he finds that he can understand the woodbird's song. Following its instructions, he takes the Ring and the Tarnhelm from Fafnir's horde. Mime reappears, and Siegfried complains that he has still not learned the meaning of fear. Mime offers him the poisoned drink. However, the dragon's blood allows Siegfried to read Mime's treacherous thoughts, and he slays the Nibelung.
The woodbird now sings of a woman sleeping on a rock surrounded by magic fire. Siegfried, wondering if he can learn fear from this woman, heads toward the mountain.
The Wanderer appears on the path to Brünnhilde's rock and summons Erda, the earth goddess. Erda, appearing confused, is unable to offer any advice. Wotan informs her that he no longer fears the end of the Gods; indeed, it is his desire. His heritage will be left to Siegfried the Walsung, and their child Brünnhilde will "work the deed that redeems the World." Dismissed, Erda sinks back into the earth.
Siegfried arrives, and the Wanderer questions the youth. Siegfried, who does not recognize his grandfather, answers insolently and starts down the path towards Brünnhilde's rock. The Wanderer blocks his path, but Siegfried breaks Wotan's spear with a blow from Nothung. Wotan calmly gathers up the pieces and vanishes.
Siegfried enters the ring of fire, emerging on Brünnhilde's rock. At first, he thinks the armored figure is a man. However, when he removes the armor, he finds a woman beneath. Uncertain about what to do, Siegfried at last experiences fear. In desperation, he kisses Brünnhilde, waking her from her magic sleep. Hesitant at first, Brünnhilde is won over by Siegfried's love, and renounces the world of the Gods. Together, they hail "light-bringing love, and laughing death."