On the banks of the river Scheldt a herald announces Heinrich, King of Germany, who asks Count Telramund to explain why Brabant is torn by strife and disorder. Telramund accuses his ward, Elsa, of murdering her brother, Gottfried, heir to Brabant’s Christian dynasty.
Gottfried was actually enchanted by the evil Ortrud, whom Telramund has wed. When Elsa is called to defend herself, she relates a dream of a knight in shining armour who will come to save her. The herald calls for the defender, but only when Elsa prays does the knight appear, magically drawn in a boat by a swan. He betrothes himself to her on condition that she never ask his name or origin. Defeating Telramund in combat, the newcomer establishes the innocence of his bride.