Brabant is in the grip of civil strife: the throne is at issue. The rightful heir, a young boy named Gottfried, has disappeared under mysterious circumstances, and his guardian, Friedrich von Telramund, intends to ascend the throne in place of Gottfried's sister, Elsa.
The German empire, of which Brabant is part, is threatened from the East, and the German king, Heinrich, has arrived in Brabant to enlist men for the war against Hungary.
King Heinrich realizes that Brabant must settle the feud over the throne, and he calls upon Telramund to explain the situation. Telramund accuses Elsa of killing her brother and claims sovereignty for himself and his wife Ortrud, a pagan princess from Friesland.
King Heinrich calls Elsa to trial, and she responds to the charges made against her with the narration of a dream promising her vindication at the hands of a mystical knight. The King and the crowd are bewildered by Elsa's vision but moved by her purity and seeming honesty. Telramund challenges to fight anyone who defends Elsa, and Elsa accepts this premise for her defense. Her faith in her vision is secure: she will marry and share the realm with her defender. The-Royal Herald calls three times for the defender to step forward.
A knight—a stranger—appears on the river, guided by a swan. He declares his love for Elsa, fights Telramund, defeats him, and spares his life, demanding that he dedicate the rest of it to contrition for his sins. The stranger's magnanimity and show of strength captivate the people's imagination, and they hail him and celebrate Elsa's vindication and innocence. Telramund is humiliated and disgraced; Ortrud wonders whether the story is, in fact, over. A mystery hangs in the air: the knight-savior has agreed to defend and wed Elsa only on condition that she never asks his name or where he’s from.
Night. Telramund and Ortrud, left behind in the celebration, wrangle angrily with each other. He reviles her: she told him that she saw Elsa kill Gottfried and her lie has ruined him. He would have married Elsa had Ortrud not seduced him with false prophecies of power, and now he has no chance at the throne. Ortrud, bent on reclaiming what has been lost, disdains Telramund's reproaches and hatches a plan. She will ingratiate herself with Elsa and then suggest that Elsa has every right to know her husband's secrets. She will demystify the knight with magic—but Telramund must bring her a part of the knight's hand, even a piece of one finger.
To Elsa, Ortrud pretends that she was shocked by Telramund's accusation and secures Elsa's promise of forgiveness for them both. Then she tries to plant a seed of doubt about the stranger in Elsa's mind. Elsa warmly protests that true love need ask no questions and welcomes Ortrud into her confidence—she. will teach her about love and faith.
At daybreak, the Herald announces that Telramund has been banished and that the stranger has been proclaimed Guardian of Brabant. Telramund enlists the help of four noblemen who express doubt about the stranger's right to the throne.
Elsa's wedding procession starts towards the church. Ortrud suddenly accosts Elsa: Why, she asks, should an unknown, unnamed stranger replace Telramund, once so highly honored and now unjustly outlawed? King and bridegroom arrive to find Elsa unnerved, and Telramund comes forward to accuse the stranger of sorcery and to ask him his name. The people stand by the stranger, but Ortrud and Telramund have reached Elsa's heart. The stranger can sense that she has begun to doubt him.
The King leads Elsa and her new husband to the bridal chamber. The stranger pours out his love to Elsa, as she does to him. But soon she suggests that he should tell her who he is: "Give me your trust, let me share your secrets, oughtn't I to see clearly who you are?" They argue passionately, and as she asks the fatal question, Telramund and the four nobles break in and attack the stranger. Elsa quickly hands her husband his sword, and he kills Telramund. The stranger orders the nobles away with Telramund's body and sends Elsa to the King, saying that he will answer her questions publicly—"All our happiness is now lost."
The men of Brabant, reunified and newly strengthened by the stranger's arrival the day before, rally around King Heinrich, ready to march toward the Hungarian front. The excitement is broken off by the appearance of Elsa and the stranger with Telramund's bier. The stranger announces that he cannot lead the men to war. He has been betrayed by Ortrud and Telramund—through Elsa. He is Lohengrin, a Knight of the Grail, who fights for virtue in the name of the Christian God, and as such is vested with supernatural powers, as long as his identity is not revealed. When it is, he must return to the Grail. He truly loves Elsa and is devastated by having to leave her.
The swan returns; Lohengrin promises Elsa that her brother will return. Ortrud exultantly confesses that she has cast a spell on Gottfried: he is the swan. But when Ortrud cries out against the Christian God, Lohengrin kneels to pray, and the swan is transformed into Elsa's lost brother Gottfried. Ortrud's spell is broken, and Lohengrin leaves Elsa and the people heartbroken at the river. Yet, there is much that is good. Elsa's name has been cleared, right has triumphed over wrong, peace has been restored to Brabant, and the true heir will take the throne. Lohengrin has fulfilled his mission.