Antwerp in the first half of the 10th century. King Heinrich der Vogler has come to Brabant to call the population to arms against the Hungarians. He asks Friedrich von Telramund why the Brabantians have no leader and are fighting amongst themselves. Telramund himself lays claims to the dukedom, maintaining that upon the Duke of Brabant’s death his daughter Elsa, in order to gain power for herself, had eliminated her brother Gottfried, heir to the title. For this reason Telramund preferred to marry Ortrud, who descends from a pagan race, rather than Elsa, even though she was promised to him by the old duke. The king then summons Elsa who, instead of defending herself against these accusations, recalls in a trance the vision of a knight sent by God, who will champion her, prove her innocence and then be her husband and sovereign. The king decides to settle the matter by ordeal of single combat. At first no champion presents himself. But after the trumpet blasts have been repeated, there appears upon the river a knight, Lohengrin, in a boat drawn by a swan. When he has stepped ashore, the knight challenges Telramund to a duel. He proclaims that he will exculpate Elsa and make her his wife. On condition, however, that she shall agree never to ask him either his name or where he comes from. Elsa accepts without hesitation. In a brief duel Lohengrin defeats Telramund but spares his life. Elsa and Lohengrin are borne in triumph through the rejoicing crowd.

The fortress in Antwerp. At night, while the festivity in honour of Elsa and Lohengrin is being held in the castle, Telramund and Ortrud are seated in a corner outside, on the steps of the church. Telramund rants at his wife, saying she is the cause of his ruin and that she, in her craving for power, deceived him and induced him to accuse Elsa unjustly. Ortrud retorts that the foreigner won the duel by magic, adding that if he were forced to reveal his name or if it were possible to cut off even the smallest part of his body, he would immediately lose all his magic powers. Ortrud convinces Telramund to work out a plan of revenge. When Elsa appears on the balcony, Ortrud orders Telramund to hide. Pretending to be sorrowful, she succeeds in arousing Elsa’s pity. Not without first having rapturously evoked the aid of the pagans to achieve her own vendetta, Ortrud shows suspicions in Elsa’s mind about the knight sent by God. In this way she also manages to get herself into the castle. Day breaks, and the herald announces the king’s wishes: that Telramund be banished and the marriage be celebrated at once between Elsa and the foreign knight, the new protector of Brabant. In a fury, Telramund wants to accuse the knight of sorcery, but four nobles for the moment keep him out of sight of the crowd. With the wedding party Elsa is on her way to the church, when Ortrud steps forward to slander her groom’s noble origins. The king and Lohengrin enter, and now it is Telramund who steps forward. He accuses the knight of sorcery and orders him to declare publicly his name and lineage. Lohengrin says he owes this answer only to Elsa. The maiden is torn by doubt and Telramund takes advantage to whisper in her ear that he is ready to stay by her that night. At a call from her, he would cut off a small portion of the knight’s body in order to break the magic spell and force the stranger to stay with her forever. Though shaken, Elsa reiterates her trust in her saviour and the bridal couple are led into the church.

The bridal chamber; the field on the banks of the Scheldt. After the wedding march, Elsa and Lohengrin are left alone. They declare their love, but whilst Elsa would like to unravel the mystery surrounding Lohengrin, the latter does his utmost to avoid the forbidden question. Just when Elsa breaks her vow not to ask it, Telramund and four noblemen burst into the room. Lohengrin slays Telramund. Aware by now that he has lost Elsa, Lohengrin tells her he will deliver his answer in public. The next morning, on the banks of the Scheldt, the king receives the Brabantians who are leaving for the war. The mysterious knight also appears, announcing that he has killed Telramund in legitimate defence and that he has been betrayed by Elsa, who asked him his name and origin. He therefore now reveals his identity. He is Lohengrin, son of Parsifal, king of the Grail. Sent out to combat evil, he is protected by a divine power, which vanishes however if he discloses his name. Neither Elsa’s entreaties nor those of the crowd persuade Lohengrin to remain. On the river by now the swan has reappeared to take the knight back whence he came. Lohengrin bids farewell to Elsa and predicts the king’s victory. Then, when Ortrud reveals that she turned Gottfried into the swan by a magic spell, Lohengrin kneels in prayer. A dove descends onto the boat, the swan vanishes into the water and re-emerges as Gottfried, ready to rule over Brabant. Ortrud falls to the ground and while Lohengrin floats away on the boat, Elsa faints into the arms of her brother.