ACT 1: It all begins with a government crisis. Before he can go to war against the Hungarians, King Heinrich must solve a leadership dispute in Brabant, following the death of the duke. At the tribunal, Friedrich von Telramund, a nobleman from Brabant who claims the dukedom, accuses Elsa, daughter of the deceased duke, of murdering her own brother Gottfried, the legitimate heir, who has mysteriously disappeared. Ortrud, wife of Telramund, has convinced her husband that she saw Elsa secretly drowning her brother while they were out walking. But nobody suspects the truth, which is that Gottfried has been transformed into a swan by Ortrud's magic. In her own defence, Elsa summons up a vision. A knight, who has appeared to her in a dream, will fight for her innocence. The king is compelled to resolve the matter by ordering a trial by single combat, relying on God's justice to determine the outcome. The royal herald's first two summonses go unanswered, but in response to a third summons, to the astonishment of all, a knight in shining armour appears in a boat pulled by a swan with a golden chain around its neck, just as Elsa's dream had foretold. The unknown knight asks Elsa whether, if he wins on her behalf, she will marry him. Elsa trusts the stranger unconditionally and agrees, accepting his enigmatic command never to ask about his origins, his lineage or his name. The stranger then defeats Telramund but spares his life.
ACT 2: The scheming continues. Telramund accuses his wife of inciting him to incriminate Elsa falsely. Ortrud cleverly exploits Telramund's pathological ambitions to advance her campaign against Elsa, whom she hates for her ability to love unconditionally. She convinces Telramund that he has fallen prey to sorcery and encourages him to seek revenge. She is even more successful at awakening Elsa's mistrust through insincerity and false empathy. Planned under the cover of darkness, her assaults on Elsa's innocence and naive trust soon take effect. At dawn, at the wedding festivities of Elsa and the stranger, Ortrud confidently steps out before her rival in front of the onlookers. Telramund publicly accuses Elsa's bridegroom of sorcery and demands that he reveal his identity. The stranger denies all the allegations against him and declares that Elsa alone may decide his fate. Elsa's faith is wavering, but she chooses to believe in her vision.
ACT 3: In the bridal chamber with her unknown knight, Elsa can no longer escape the confession of her longing. The rift between the lovers becomes insurmountable; Elsa needs to know who her husband is if she is to remain true to herself. The overpowering impulse to know the knight's identity is shared by Telramund, who bursts into the bridal chamber before the knight has had time to reply. The knight kills Telramund and commands that his body be brought before the king for judgement. Once there, he justifies the killing of Telramund and then reveals his identity: he is Lohengrin, son of Parsifal, Knight of the Holy Grail. Now that he has had to give up his secret, he can no longer remain in Brabant and cannot lead the army against the invaders; he must return home. The swan reappears and, as Lohengrin bids her farewell, he tells Elsa that with her lack of faith, she has sealed the fate of her brother, whom she had believed to be dead: after a year of untroubled happiness between the two lovers, Gottfried would have become human again. Ortrud savours her triumph and admits to having enchanted Gottfried, but her magic has served its purpose and with her confession she has condemned herself. The swan disappears and is replaced by Gottfried, whom Lohengrin names as the new ruler of Brabant, before returning to the Grail. Elsa, calling for her lost husband, dies in her brother's arms.
