Here is a brief outline of the libretto:—
Ernani, an Italian rebel of obscure parentage is the accepted lover of Donna Elvira, the high-born niece of Don Ruy Gomez de Silva, Grandee of Spain.
Donna Elvira is also coveted by Don Carlos, King of Spain, and by her old uncle Silva, who is about to wed her, much against her will.
Ernani comes to Silva's castle in the garb of a pilgrim, and finds the King in Donna Elvira's room, trying to lure her away. Here they are surprised by de Silva, who, failing to recognize his sovereign challenges both men to mortal combat.—When he recognizes the King in one of his foes, he is in despair and humbly craves his pardon, which is granted to him.—At the same time Don Carlos sends Ernani away on a distant errand, hoping to rid himself of him once for all; but Donna Elvira vows to kill herself rather than belong either to the King or to her uncle, and promises unwavering constancy to her lover Ernani.
Nevertheless the second Act shows Elvira on the eve of her wedding with her uncle de Silva.
Ernani, once more proclaimed an outlaw seeks refuge in de Silva's castle, again disguised as a pilgrim. But when Ernani hears of Donna Elvira's approaching marriage with de Silva, he reveals his identity and offers his head to the old man, telling him that his life is forfeited and that a reward is offered for his capture. De Silva is too generous to betray his rival; he orders the gates of the castle to be barred at once.—While this is being done, Ernani violently reproaches Elvira for having played him false. She answers, that she has been led to believe him dead, and dissolved in tears they embrace tenderly. Thus they are surprised by de Silva who, though for the time being bound by the laws of hospitality swears to destroy Ernani, wherever he may find him.
For the moment however he conceals his foe so well, that Don Carlos' followers cannot find him. Though the King threatens to take the old man's life, the nobleman remains true to his word and even makes the greatest sacrifice by delivering Elvira as a hostage into the King's hands.
Left alone he opens Ernani's hiding-place and challenges him to fight, but when the latter proves to him, that Don Carlos is his rival and wants to seduce Elvira, de Silva's wrath turns against the King.
He accepts Ernani's offer to help him in frustrating the King's designs, but at the same time he reminds him that his life is forfeited.—Ernani declares himself satisfied and gives de Silva a bugle, the sound of which is to proclaim, that the hour of reckoning between the two foes has come.