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.
That same night Mina, overwhelmed with remorse, seeks comfort on her mother's tomb: imploringly, she prays to her to help her and to ask God to forgive her. Godvino discovers her there: even though Mina implores him not to profane a sacred place, he declares his love to her. She repulses him, asking him to give her back her ring; when he obstinately refuses, she threatens to tell everything to her husband. But Egberto arrives and commands once again that his daughter not reveal the truth to Aroldo. Then he challenges Godvino to a duel. At first he refuses to fight with an old man, but, provoked by Egberto's insults, he accepts. Hearing the sound of clashing swords, Aroldo rushes in and orders to two men to stop. In an attempt to reconcile them, he makes Godvino, the younger, throw down his sword first, then disarms him and gives him his hand. Horrified, Egberto tells his son-in-law that he has given his hand to the man who betrayed him. Surprised, Aroldo asks Mina, who has arrived on the scene, to affirm her innocence, but at the woman's obstinate silence, he takes Egberto's sword and is about to attack Godvino, when he hears the chorus of the faithful singing the Miserere in the church. Briano arrives and reminds his friend that a good Christian must be ready to forgive: dragging himself to the foot of the cross, Aroldo collapses.
Egberto learns that Godvino has fled and left a letter for his daughter in which he begs her to follow him. Oppressed by the shame of being unable to avenge his family's honour, Egberto is contemplating ending his own life when Briano comes to tell him that Godvino has been captured and is being brought back to the castle. Egberto, already tasting sweet revenge, abandons himself to overflowing joy. Aroldo enters with Godvino: the crusader asks him what he would do if Mina were free of conjugal constraints, but the other cannot believe such a thing possible. Aroldo makes him go into the other room from where he will be able to hear, unseen, Aroldo's conversation with his wife. She is summoned, and Aroldo tells her that since the basis of their union, that is, love, is no longer felt, they should part, and gives her a request for divorce to sign. In tears, she at first resists, but then, annoyed by her husband's reproaches, she accepts. But now that he is no longer her husband, she asks him to hear her confession as her judge: she had been tricked into adultery, but in her heart she had always been faithful. Aroldo, touched, is not sure whether to punish Godvino, but at that moment, Egberto arrives with a bloody sword: he himself has killed the traitor. Briano and Aroldo go to pray in church, while Mina once more asks for divine forgiveness.
A valley in Scotland. It is evening. Shepherds, women and hunters are coming home from the hills singing: Aroldo and Briano also return to their modest dwelling, where they now live far from worldly things: the serenity of the place contrasts with Aroldo's torment, still in love with his wife. As soon as they hear the church bells chime the Ave Maria the two men kneel to pray, then go into the house. A strong wind rises, agitating the waters of the nearby Lake Lomond, announcing an approaching storm. The tempest breaks just as a boat is approaching the shore. The people of the village hurry to throw a line to pull it to safety; after several attempts, it manages to tie up to shore. Out come Mina and Egberto. Looking for refuge, they knock at Aroldo's door; he opens, and seeing his wife, tries to push her away, but Mina begs him to forgive her. Egberto implores his pity as well. Once more Briano reminds his friend of his Christian duties, encouraging him to forgive her. As if inspired by the heavens, Aroldo forgives Mina. The two embrace: the divine law of love has triumphed.
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.