Verdi composed the work for the San Carlo Theatre, Naples, where it was to have been produced for the carnival of 1858. But January 14th of that year, and while the rehearsals were in progress, Felice Orsini, an Italian revolutionist, made his attempt on the life of Napoleon III. In consequence the authorities forbade the performance of a work dealing with the assassination of a king. The suggestion that Verdi adapt his music to an entirely different libretto was put aside by the composer, and the work was withdrawn, with the result that a revolution nearly broke out in Naples. People paraded the street, and by shouting "Viva Verdi!" proclaimed, under guise of the initials of the popular composer's name, that they favoured the cause of a united Italy, with Victor Emanuel as King; viz.: Vittorio Emmanuele Re D'Italia (Victor Emanuel, King of Italy). Finally the censor in Rome suggested, as a way out of the difficulty, that the title of the opera be changed to "Un Ballo in Maschera" and the scene transferred to Boston. For however nervous the authorities were about having a king murdered on the stage, they regarded the assassination of an English governor in far-off America as a quite harmless diversion. So, indeed, it proved to be, the only excitement evinced by the audience of the Apollo Theatre, Rome, on the evening of February 18, 1859, being the result of its enthusiasm over the various musical numbers of the work, this enthusiasm not being at all dampened by the fact that, with the transfer to Boston, two of the conspirators, Samuel and Tommaso, became negroes, and the astrologer who figures in the opera, a negress.
The sensible change of scene from Boston to Naples is said to have been initiated in Paris upon the instance of Mario, who "would never have consented to sing his ballad in the second act in short pantaloons, silk stockings, red dress, and big epaulettes of gold lace. He would never have been satisfied with the title of Earl of Warwick and the office of governor. He preferred to be a grandee of Spain, to call himself the Duke of Olivares, and to disguise himself as a Neapolitan fisherman, besides paying little attention to the strict accuracy of the rôle, but rather adapting it to his own gifts as an artist." The ballad referred to in this quotation undoubtedly is Richard's barcarolle, "Di' tu se fedele il flutto m'aspetta" (Declare if the waves will faithfully bear me).
Act I. Reception hall in the Governor's house. Richard, Earl of Warwick, is giving an audience. Oscar, a page, brings him the list of guests invited to a masked ball. Richard is especially delighted at seeing on it the name of Amelia, the wife of his secretary, Reinhart, although his conscience bitterly reproaches him for loving Amelia, for Reinhart is his most faithful friend, ever ready to defend him. The secretary also has discovered a conspiracy against his master; but as yet has been unable to learn the names of the conspirators.
At the audience a judge is announced, who brings for signature the sentence of banishment against an old fortune teller, the negress Ulrica. Oscar, however, intercedes for the old woman. Richard decides to visit her in disguise and test her powers of divination.
The scene changes to Ulrica's hut, which Richard enters disguised as a fisherman. Without his knowledge, Amelia also comes to consult the negress. Concealed by a curtain he hears her ask for a magic herb to cure her of the love which she, a married woman, bears to Richard. The old woman tells her of such an herb, but Amelia must gather it herself at midnight in the place where stands the gibbet. Richard thus learns that she loves him, and of her purpose to be at the place of the gibbet at midnight. When she has gone he comes out of his concealment and has his fortune told. Ulrica predicts that he will die by the hand of a friend. The conspirators, who are in his retinue, whisper among themselves that they are discovered. "Who will be the slayer?" asks Richard. The answer is, "Whoever first shall shake your hand." At this moment Reinhart enters, greets his friend with a vigorous shake of the hand, and Richard laughs at the evil prophecy. His retinue and the populace rejoice with him.
Act II. Midnight, beside the gallows. Amelia, deeply veiled, comes to pluck the magic herb. Richard arrives to protect her. Amelia is unable to conceal her love for him. But who comes there? It is Reinhart. Concern for his master has called him to the spot. The conspirators are lying in wait for him nearby. Richard exacts from Reinhart a promise to escort back to the city the deeply veiled woman, without making an attempt to learn who she is, while he himself returns by an unfrequented path. Reinhart and his companion fall into the hands of the conspirators. The latter do not harm the secretary, but want at least to learn who the Governor's sweetheart is. They lift the veil. Reinhart sees his own wife. Rage grips his soul. He bids the leaders of the conspiracy to meet with him at his house in the morning.
Act III. A study in Reinhart's dwelling. For the disgrace he has suffered he intends to kill Amelia. Upon her plea she is allowed to embrace her son once more. He reflects that, after all, Richard is much the more guilty of the two. He refrains from killing her, but when he and the conspirators draw lots to determine who shall kill Richard, he calls her in, and, at his command, she draws a piece of paper from an urn. It bears her husband's name, drawn unwittingly by her to indicate the person who is to slay the man she loves. Partly to remove Amelia's suspicions, Reinhart accepts the invitation to the masked ball which Oscar brings him, Richard, of course, knowing nothing of what has transpired.
In the brilliant crowd of maskers, the scene having changed to that of the masked ball, Reinhart learns from Oscar what disguise is worn by Richard. Amelia, who, with the eyes of apprehensive love, also has recognized Richard, implores him to flee the danger that threatens him. But Richard knows no fear. In order that the honour of his friend shall remain secure, he has determined to send him as an envoy to England, accompanied by his wife. Her, he tells Amelia, he will never see again. "Once more I bid thee farewell, for the last time, farewell."
"And thus receive thou my farewell!" exclaims Reinhart, stabbing him in the side.
With his last words Richard assures Reinhart of the guiltlessness of Amelia, and admonishes all to seek to avenge his death on no one.