The text, by Antonio Somma, was based on Eugène Scribe's libretto for Daniel Auber's 1833 five act opera, Gustave III, ou Le bal masqué.

[1] The plot concerns the assassination in 1792 of King Gustav III of Sweden[2] who was shot, as the result of a political conspiracy, while attending a masked ball, dying of his wounds thirteen days later.

In becoming the Un ballo in maschera which we know today, Verdi's opera (and his libretto) underwent a significant series of transformations and title changes, caused by a combination of censorship regulations in both Naples and Rome, as well as by the political situation in France in January 1858.

A commission by the Teatro San Carlo in Naples in early 1857 led Verdi to begin to oversee the finalization of the libretto (also by Somma) for Re Lear with the aim of presenting the finished opera during the 1858 carnival season.

When this proved to be impracticable, Verdi turned to the subject of King Gustav III's assassination as portrayed in Scribe and Auber's opera, albeit not an historically accurate narrative.

For the libretto, Scribe retained the names of some of the historical figures involved (including fortune teller Ulrica Arfvidsson, the conspiracy, and the killing at the masked ball, but, as noted by Budden, "it was a simple case of 'cherchez la femme'": for the rest of the play Scribe invented the romance between the King and the fictional Amélie,[3] the wife of the king's secretary and best friend, and adds characters and situations such as Oscar, the page boy.

[5] As had happened with Rigoletto, changes in characters' names and titles were proposed (the King of Sweden became the Duke of Pomerania; Anckarström became Count Renato) and the location was moved from Stockholm to Stettin.

[5] The composer then travelled to Naples and rehearsals of Una vendetta were about to begin when, on 14 January 1858, three Italians attempted to assassinate Emperor Napoleon III in Paris, an event which was to affect the opera's production.

It was during this period of turmoil that Verdi was to describe the previous sixteen years of his composing life: in a letter to Countess Clara Maffei, he states: "From Nabucco, you may say, I have never had one hour of peace.

"[7][8] When the legal issues were resolved within a few months, Verdi was free to present the libretto and musical outline of Gustave III (which was basically Una vendetta with characters' names and locations changed)[9] to the Rome Opera.

Removing the action from Europe, the location became Boston during the British colonial period and the leading character became Riccardo, the Count (or Earl) of Warwick.

[11] On 7 January 1955,[12] Marian Anderson, singing the role of Ulrica, broke the "color barrier" at the Metropolitan Opera, becoming the first African-American vocal soloist to appear with that company.

[16] David Richards has argued that although the opera was no longer explicitly based on Gustav III, Verdi deliberately deviated from his usual practice and set Oscar for a soprano – despite disliking women singing men's parts: "Verdi goes as far as one could go within the repressive conventions of his period to portray Gustavo (based on a widely known flamboyantly homosexual ruler) as either a gay man or, at a minimum, a bisexual man".

He makes the fortune of a sailor named Silvano come true by spiriting a document of promotion into his pouch, convincing the crowd of the truth of Ulrica's powers.

Renato relents, and declares that it is Riccardo, not Amelia, who deserves to die (Aria: Eri tu che macchiavi quell'anima / "You were the one who stained her soul").

The prelude to the piece is composed of themes from the opening scene of the opera, with dramatic contrast between the music of Riccardo's devoted courtiers and the dark mutterings of those who hate him.

[21] Influence of French opera is felt in the first scene, both in the vocal writing for Oscar the page-boy, sprinkled with coloratura effects, and in the closing ensemble in which Riccardo invites everyone to visit the witch's den in disguise.

The music of this ensemble combines French-style elan and a sense of joie de vivre with continued low grumblings from those plotting Riccardo's assassination.

Riccardo's light-hearted character is developed in the aria he sings in the disguise of a fisherman Di' tu se fedele, with the characteristics of a barcarolle.

The grand scene for the soprano which follows is multi-sectioned with a slow cantilena accompanied by solo cor anglais, then a dramatic and quick middle section as Amelia is terrified by an apparition, and a slower anguished prayer with a final cadenza calling for an exceptional range from high to low.

A quartet with chorus closes the act, with radically different emotions once again being conveyed simultaneously in a single piece of music – the sardonic, mocking laughter of the conspirators, the anguish of Amelia and the implacable hatred and desire for revenge of her husband.

[21] The music of the aria for Riccardo that opens the final scene delineates the character's evolution from a selfish and light-hearted pleasure seeker to a serious man who will sacrifice his own happiness in order to do what he now recognises as the right thing.

[21] The opera is scored for flute, piccolo, two oboes, cor anglais, two clarinets, two bassoons, four horns, two trumpets, three trombones, cimbasso, timpani, cymbals, bass drum, harp and strings, together with offstage wind band, offstage bell and small onstage string orchestra (four to six first violins, two second violins, two violas, two cellos and two double basses).