A cave in rocks in the forest. An orchestral introduction includes references to leitmotifs including themes relating to the original hoard plundered by the Nibelung Alberich, and one in B-flat minor associated with the Nibelungs themselves. As the curtain rises, Alberich's brother, the dwarf Mime, is forging a sword. Mime is plotting to obtain the ring of power originally created by his brother Alberich. He has raised the human boy Siegfried as a foster child, to kill Fafner, who obtained the ring and other treasures in the opera Das Rheingold and has since transformed himself from a giant to a dragon. Mime needs a sword for Siegfried to use, but the youth has contemptuously broken every sword Mime has made. Siegfried returns from his wanderings in the forest with a wild bear in tow, and immediately breaks the new sword. After a whining speech by Mime about ingratitude, and how Mime has brought him up from a mewling infant, Siegfried senses why he keeps coming back to Mime although he despises him: he wants to know his parentage. Mime is forced to explain how he took in Siegfried's mother, Sieglinde, who then died giving birth to Siegfried. He shows Siegfried the broken pieces of the sword Nothung, which Mime had obtained from her. Siegfried orders him to reforge the sword; Mime, however, is unable to accomplish this. Siegfried departs, leaving Mime in despair.
The Wanderer (Wotan in disguise) arrives at the door and introduces himself as the Wanderer. In return for the hospitality due a guest, he wagers his head on answering any three questions of Mime. The dwarf asks the Wanderer to name the races that live beneath the ground, on the earth, and in the skies. These are the Nibelung, the Giants, and the Gods, as the Wanderer answers correctly. The Wanderer then induces Mime to wager his own head on three further riddles: the race most beloved of Wotan, but most harshly treated; the name of the sword that can destroy Fafner; and the person who can repair the sword. Mime answers the first two questions: the Wälsungs (Siegmund and Sieglinde) and Nothung. However, he cannot answer the last. 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 Siegfried is "the one who does not know fear" and that unless he can instill fear in him, Siegfried will kill him as the Wanderer foretold. He tells Siegfried that fear is an essential craft; Siegfried is eager to learn it, and Mime promises to teach him by taking him to Fafner. 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. After he finishes forging the sword, Siegfried demonstrates its strength by chopping the anvil in half with it.
Deep in the forest. The Wanderer arrives at the entrance to Fafner's cave, where Alberich is keeping vigil. The two enemies recognize each other. Alberich boasts of his plans to regain the ring and rule the world. Wotan states that he does not intend to interfere, only to observe. He even offers to awaken the dragon so that Alberich can bargain with him. Alberich warns the dragon that a hero is coming to kill him, and offers to prevent the fight in exchange for the ring. Fafner dismisses the threat, declines Alberich's offer, and returns to sleep. Wotan leaves and Alberich withdraws, muttering threats.
At daybreak, Siegfried and Mime arrive. After assuring Siegfried that the dragon will teach him what fear is, Mime withdraws. As Siegfried waits for the dragon to appear, he hears a woodbird singing. 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 Fafner out of his cave. After a short exchange, they fight; Siegfried stabs Fafner in the heart with Nothung. In his last moments, Fafner learns Siegfried's name, and tells him to beware of treachery. When Siegfried withdraws his sword from Fafner's body, his hands are burned by the dragon's blood and he puts his finger in 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 magic helmet Tarnhelm from Fafner's hoard.
Outside the cave, Alberich and Mime quarrel over the treasure. Alberich hides as Siegfried comes out of the cave. Siegfried complains to Mime that he has still not learned the meaning of fear. Mime offers him the poisoned drink; however, the magic power of the dragon's blood allows Siegfried to read Mime's treacherous thoughts, and he stabs him to death. He throws Mime's body into the treasure cave and places Fafner's body in the cave entrance to block it. 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, follows the bird towards the rock.
At the foot of Brünnhilde's rock. The Wanderer 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 Wälsung, and Brünnhilde (Erda's and Wotan's child), who 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 toward Brünnhilde's rock. The Wanderer blocks his path, but Siegfried mocks him, laughing at his floppy hat and his missing eye, and breaks his spear (the symbol of Wotan's authority) with a blow from Nothung. Wotan calmly gathers up the pieces and vanishes.
Siegfried passes through the ring of fire, emerging on Brünnhilde's rock. At first, he thinks the sleeping armored figure is a man. However, when he removes the armor, he finds a woman beneath. At the sight of the first woman he has ever seen, 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.
