As a storm rages, Siegmund the Walsung, exhausted from pursuit by enemies in the forest, stumbles into an unfamiliar house for shelter. Sieglinde finds the stranger lying by the hearth, and the two feel an immediate attraction. But they are soon interrupted by Sieglinde's husband, Hunding, who asks the stranger who he is. Calling himself "Woeful," Siegmund tells of a disaster-filled life ("Friedmund darf ich nicht heissen"), only to learn that Hunding is a kinsman of his foes. Hunding, before retiring, tells his guest to defend himself in the morning. Left alone, Siegmund calls on his father, Walse, for the sword he once promised him. Sieglinde reappears, having given Hunding a sleeping potion. She tells of her wedding, at which a one-eyed stranger thrust into a tree a sword that thereafter resisted every effort to pull it out ("Der Manner Sippe"). Sieglinde confesses her unhappiness to Siegmund, whereupon he ardently embraces her and vows to free her from her forced marriage to Hunding. As moonlight floods the room, Siegmund compares their feeling to the marriage of love and spring ("Wintersturme"). Sieglinde hails him as "Spring" ("Du bist der Lenz") but asks if his father was really "Wolf," as he said earlier. When Siegmund gives his father's name as Walse instead, Sieglinde rapturously recognizes him as Siegmund, her twin brother. The Walsung now draws the sword from the tree and claims Sieglinde as his bride, rejoicing in the union of the Walsungs.
High in the mountains, Wotan, leader of the gods, tells his warrior daughter Brünnhilde she must defend his mortal son Siegmund. Leaving joyfully to do his bidding ("Hojotoho!"), the Valkyrie pauses to note the approach of Fricka, Wotan's wife and the goddess of marriage. Fricka insists he must defend Hunding's marriage rights against Siegmund, ignoring Wotan's implied argument that Siegmund could save the gods by winning back the Rhinegold from the dragon Fafner before the Nibelung dwarfs regain it. When Wotan realizes he is caught in his own trap - his power will leave him if he does not enforce the law - he agrees to his wife's demands. After Fricka has left in triumph, the frustrated god tells the returning Brünnhilde about the theft of the gold and Alberich's curse on it ("Als junger Liebe"). Brünnhilde is shocked to hear her father, his plans in ruins, order her to fight for Hunding. Then, alone in the darkness, she withdraws as Siegmund and Sieglinde approach. Siegmund comforts the distraught girl, who feels herself unworthy of him, and watches over her when she falls asleep.
Brünnhilde appears to him as if in a vision, telling him he will soon go to Valhalla (Todesverkundigung: "Siegmund! Sieh auf mich!"), but when he says he will not leave Sieglinde and threatens to kill himself and his bride if his sword has no power against Hunding, she decides to help him in spite of Wotan's command. She vanishes. Siegmund bids farewell to Sieglinde when he hears the approaching Hunding's challenge. When Siegmund is about to win, however, Wotan appears and shatters his sword, leaving him to be killed by Hunding. Brünnhilde escapes with Sieglinde and the broken sword. Wotan contemptuously fells Hunding with a wave of his hand and leaves to punish Brünnhilde.
On the Valkyries' Rock, Brünnhilde's eight warrior sisters - who have gathered there briefly, bearing slain heroes to Valhalla - are surprised to see her enter with Sieglinde. When they hear she is fleeing Wotan's wrath, they are afraid to hide her. Sieglinde is numb with despair until Brünnhilde tells her she bears Siegmund's child. Eager to be saved, she receives the pieces of the sword from Brünnhilde and ecstatically thanks her rescuer as she rushes off into the forest to hide near Fafner's cave, a place safe from Wotan. When the god appears, he sentences Brünnhilde to become a mortal woman, silencing her sisters' objections by threatening to do the same to them.
Left alone with her father, Brünnhilde pleads that in disobeying his orders she was really doing what he wished ("War es so schmahlich"). Wotan will not relent: she must lie in sleep, booty for any man who finds her. But as his anger abates she asks the favor of being surrounded in sleep by a wall of fire that only the bravest hero can pierce. Both sense this hero must be the child that Sieglinde will bear. Sadly renouncing his daughter ("Leb' wohl"), Wotan kisses Brünnhilde's eyes with sleep and mortality before summoning Loge, the spirit of fire, to encircle the rock. As flames spring up, the departing Wotan invokes a spell forbidding the rock to anyone who fears his spear (Fire Music).