Wagner used the poem "Parzival" by Wolfram von Eschenbach as the source for the plot. The material, borrowed from sagas, legends and poems, was creatively revised to create an original concept that reflects contemporary philosophical and ethical concerns.
Count Telramund, who seeks to acquire the Principality of Brabant, accuses the Brabant princess Elsa of having killed her brother. King Heinrich decides that Elsa's guilt must be established by a so-called "God's judgment", in which the accuser enters into a duel with a knight defending the honour of the accused. The royal herald appeals to the knights to intercede in Elsa's defence, while in the meantime she pleads with heaven for help. At the second summons a swan appears towing a boat with a knight. This knight takes over Elsa's defence and defeats Telramund, but spares his life. As a reward for his feat, he asks for Elsa's hand in marriage, but makes one condition – she must not ask him who he is or where he came from. If the vow is broken, they will be parted forever. Elsa agrees.
The defeated Telramund and his wife Ortrud, an evil sorceress, are banished by the King, but return to take revenge on Elsa and the unknown knight. Ortrud wants to destroy Elsa and to this end seeks to instil in her a distrust of her husband. As Elsa and Lohengrin's wedding cortege approaches the temple, Ortrud blocks the entrance and declares that she, Count Telramund's wife, must enter first, as Elsa's husband's name is known to no one. Before the sight of Lohengrin, Ortrud falls silent. But Telramund insists that the knight tell his name. The unknown is silent, awaiting Elsa's answer. She endures the ordeal, but the first doubts creep into her soul.
After the marriage ceremony, Elsa breaks her promise and asks her husband to tell his name and lineage. At that moment Telramund rushes into the chambers and tries to kill the knight, but is struck down by his sword. Saddened by Elsa's infidelity, the knight promises to reveal his name.
Before the King and the people, he announces that his name is Lohengrin, son of Parsifal, the leader of the community of the Knights of the Holy Grail, which is located on the inaccessible Mount Monsalvat, and has come to earth to save an innocent maiden. He can remain among men as long as faith in him lives. Now Lohengrin must part with Elsa.
A white boat pulled by a swan arrives on the river. The knight returns the swan to the form of a young man – this is Gottfried, Elsa's supposedly dead brother, transformed into a bird by the magic of Ortrud. A white dove flies in to take away Lohengrin, who boards the boat. Elsa falls breathless into her brother's arms.
