Hall in the castle of Egberto in Kent. The old knight Egberto has organised a banquet in honour of Aroldo, his son-in-law, who has just come home from the crusade in Palestine. Inside are heard songs celebrating Aroldo's victory over the Saracens. Greatly agitated, Mina, daughter of Egberto and wife of Aroldo, rushes out of the banquet hall: while her husband was at war, she has been unfaithful, letting herself be seduced by Godvino, one of her father's guests. Now that Aroldo has returned she is troubled by remorse and prays heaven for help. Aroldo joins her, accompanied by the pious Briano, who has become his inseparable friend ever since he saved Aroldo's life in Palestine. Once they are alone, Aroldo tells Mina how he thought of her constantly while he was away. These words only increase Mina's sense of guilt, and she bursts into tears. Aroldo is surprised that she no longer wears her wedding ring and asks her where it is: she does not answer. Briano calls him and Aroldo leaves. Mina, alone, despairs with her head in her hands. Unseen, Egberto arrives; he realises from his daughter's malaise that his suspicions about Godvino must be true. Mina decides to write a letter confessing all to her husband, but is stopped by her father, who advises her not to send anything to Aroldo if she does not want to kill him with grief.
Godvino, seeing that Mina is ignoring him, while he burns with desire for her, decides to write a letter to her. This he leaves in a book closed by a clasp of which he has the key; he puts the book on the table. Briano sees all this from a distance, thinking that Godvino is a friend of Aroldo's. The hall is filling up with guests. Among these is Enrico, Mina's cousin. Briano, convinced that Aroldo's honour is being compromised, tells his friend what he has seen, but erroneously identifies the man who put the letter in the book as Enrico, who is dressed almost the same as Godvino. Aroldo has trouble containing his fury. In the meantime the guests crowd around him, congratulating him, and Egberto asks him to tell of the feats of King Richard in Palestine. But Aroldo prefers to tell the story of a man who, hiding a letter in a book, insidiously abused the honour of a friend; a similar story, he goes on, is told in that book there on the table, and asks Mina for the key to open it. The guests are bewildered: when the woman refuses, Aroldo breaks the clasp and the letter falls to the floor. Egberto picks it up but refuses to give it to his son-in-law. Aroldo curses the old man, even though Mina bids him respect his elder. Egberto, meanwhile, whispers to Godvino to join him later at the cemetery; he will challenge him to a duel.
This is a remake of another Verdi opera, Stiffelio (1850) that had brought on the intervention of the censors because of the "immoral and indecent" story of a Protestant minister betrayed by his wife; moreover, the German characters did not inspire Italian audiences. In 1856 Verdi decided to rewrite the opera, with the help of his friend and librettist Francesco Maria Piave gaining inspiration from two novels: The Betrothed by Walter Scott and Harold by Bulwer-Lytton. He made substantial corrections to several scenes of the three acts of Stiffelio (the first version) and added a new fourth act. With a year of work the opera was ready to go on stage, and Verdi chose Bologna as the venue for the first night, but at the behest of his friend and publisher Ricordi, Rimini was to be the place. The Marzi brothers, impresarios for Reggio Emilia and Rimini, were looking for a new opera for the inauguration of the Teatro Nuovo in Rimini. The evening of August 16th, Aroldo was warmly applauded by a full house, to be presented later in Bologna, as the composer had at first intended, and then Turin and Naples.