Brabant expects the arrival of King Heinrich (“Henry the Fowler”) to pronounce judgment between Friedrich von Telramund and Elsa of Brabant, and thereby to ease a political crisis in a part of his domain threatened by a foreign power. Elsa, heir to the Brabantine throne, is accused by Telramund of having murdered her own brother in order to succeed to the throne herself together with some unknown lover. Elsa is summoned to judgment; she tells of a knight of whom she has had a vision and who will champion her cause. On a threefold summons a stranger duly appears to the fascination of all. He declares himself willing to defend Elsa and asks for her hand. But he makes a stipulation: she may never ask what his name is and where he comes from. In combat with Telramund the stranger emerges as the victor; he is acclaimed as a hero and as harbinger of a stabilized future.
Telramund and his wife Ortrud are outcast and in disgrace. He accuses her of having driven him to make a false accusation. But Ortrud has thoughts only for how she can still grasp power. She plans to persuade Elsa to put the forbidden question to her new champion. But Elsa’s love is for the moment stronger.Elsa’s forthcoming wedding to her knight, who wishes the title “Protector of Brabant”, is announced, and also, that the unknown hero will be the commander of the imminent military operation. Telramund is in search of allies who can thwart the Protector’s rise to power. The wedding procession is interrupted by Ortrud’s claiming precedence over her. Ortrud stresses her own blameless standing and the unclarified origin of Elsa’s hero. While Elsa looks to the latter for support, Telramund then also appears and accuses him of witchcraft; his rival’s reply is that that is only for Elsa to judge. While Telramund tries to undermine Elsa’s loyalty, the crowd applauds the wedding couple.
In the bridal chamber the newlyweds are alone for the first time. Elsa has begun to doubt that she can be sure of the stranger’s loyalty for ever, and is driven to demand of him to reveal his identity. During this confrontation Telramund bursts in and, making an attempt on his rival’s life, is killed. The Protector orders that the corpse should be brought before the King, where he will answer for himself. The King and his entourage are waiting for the Protector of Brabant and are confronted by the corpse of Telramund. To vindicate this killing the stranger feels constrained to explain his identity: he is Lohengrin, sent by the companionship of the Holy Grail as an anonymous champion of rectitude. But because Elsa has failed to trust his anonymity, the revelation of his secret forces him to leave Brabant and to return to the Grail. Ortrud triumphs, Lohengrin slips away and with him the hopes of a new political security for Brabant.