The action of the opera takes place in Arragon, Spain, and the period is 1519.  Elvira, a noble Spanish lady, betrothed to the grandee Don Gomez de Silva, is in love with the bandit Ernani, who forms a plan to carry her off.  While receiving the congratulations of her friends upon her approaching marriage with Silva, Don Carlos, the King of Spain, enters her apartment, declares his passion for her, and tries to force her from the castle.  She cries for help, and Ernani comes to her rescue and defies the king.  The situation is still further complicated by the sudden arrival of Silva, who declares he will avenge the insult.  Finding, however, that it is the King whom he has challenged, he sues for pardon.  In the second act, as the nuptials are about to be solemnized, Ernani enters, disguised as a pilgrim, and believing Elvira false to him, throws off his disguise and demands to be given up to the King, which Silva refuses, as he cannot betray a guest.  Discovering, however, that Elvira and Ernani are attached to each other, he determines on vengeance.  The King eventually carries off Elvira as a hostage of the faith of Silva, whereupon the latter challenges Ernani.  The bandit refuses to fight with him, informs him that the King is also his rival, and asks to share in his vengeance, promising in turn to give up his life when Silva calls for it, and presenting him with a horn which he is to sound whenever he wishes to have the promise kept.  In the third act, the King, aware that the conspirators are to meet in the catacombs of Aquisgrana, conceals himself there, and when the assassins meet to decide who shall kill him, he suddenly appears among them and condemns the nobles to be sent to the block.  Ernani, who is a duke, under the ban of the King of Castile, demands the right to join them, but the King magnanimously pardons the conspirators and consents to the union of Ernani and Elvira.  Upon the very eve of their happiness, and in the midst of their festivities, the fatal horn is heard, and true to his promise Ernani parts from Elvira and kills himself.

The first act opens with a spirited chorus of banditti and mountaineers ("Allegri, beviami”) as they are drinking and gambling in their mountain retreat.  Ernani appears upon a neighboring height and announces himself in a despondent aria ("Come rugiada al cespite").  A brief snatch of chorus intervenes, when he breaks out in a second and more passionate strain ("Dell’ esilio nel dolore"), in which he sings of his love for Elvira.  The third scene opens in Elvira’s apartments, and is introduced with one of the most beautiful of Verdi’s arias, “Ernani, involami,” with which all concert-goers have become acquainted by its frequent repetition.  A graceful chorus of her ladies bearing gifts leads to a second and more florid number ("Tutto sprezzo che d’ Ernani").  Don Carlos enters, and in the seventh scene has an aria ("Bella come un primo amore”) in which he declares his passion for Elvira, leading up to a very dramatic duet between them ("Fiero sangue d’ Aragona").  This is followed in turn by a trio between the two and Ernani.  The finale commences with an impressive and sonorous bass solo ("Infelice! e tuo credevi”) by Silva, and closes with a septet and chorus of great power.