The drama takes place in 1519 Spain where the bandit Don John of Aragon, aka Ernani, in order to claim the murder of his father, is leading a revolt against King Charles.
Ernani’s revolt fails and the rebel, disguised as a pilgrim, takes refuge with Silva.
Silva, great of Spain and Elvira’s uncle, is also the woman’s betrothed. King Charles is also in love with Elvira, but she reciprocates the feeling only with Ernani.
King Charles, in pursuit of Ernani, arrives at the castle, but unable to find Ernani, asks Elvira to follow him. In order to protect Elvira, Ernani in despair decides to tell Silva that King Charles is also in love with his niece and asks for help in vindicating Elvira’s offended honor.
Silva agrees and Ernani sanctions the pact by giving him a hunting horn: when Silva wants Ernani dead, he will blow the horn three times and Ernani will take his own life.
In the third act, set in the dungeon of the Aachen chapel, right on the tomb of Charlemagne, the conspirators gather, led by Ernani, but anticipated precisely by Charles who claims the imperial throne and decrees the death of the two: Ernani and Silva .
At this point Elvira intervenes in defense of Ernani, who pleads with Carlo to spare her lover and her uncle.
Charles relents, even agreeing to the marriage between Ernani and Elvira.
At this point, however, irate Silva meditates revenge for his just-shortened marriage to the woman.
The fourth act thus begins on the terrace in Ernani’s palace, where the bride and groom’s party is in progress. At one point, however, one of the servants notices a man dressed in a black cloak and masked, with an angry look and a stealthy attitude.
And at the moment when Elvira and Ernani finally exchange eternal love, the sound of a horn is heard. Ernani instantly understands, mindful of the oath made with Silva, and pushes Elvira away. With his mask off, Silva unveils himself and reminds Ernani of the pact.
Ernani, true to his word, is forced to keep his oath, so he grabs a dagger and kills himself.