This is the story of Beatrice Lascaris di Tenda, the woman who was the widow of the condottiere Facino Cane and later the wife of Duke Filippo Maria Visconti, in 15th century Milan. Filippo has grown tired of his wife Beatrice; she regrets her impetuous marriage to him after her first husband's death, a marriage that has delivered her and her people into the Duke's tyrannical power.
Filippo attends a ball at the Castle Binasco in Italy, shadowed as usual by the sinister Rizzardo. He is fed up with everyone paying obeisance to his wife. His sycophantic courtiers tell him how much they sympathize, and suggest that Beatrice's servants are all plotting against him.
Beautiful harp music is heard. Agnese, the current object of Filippo's lust, sings from afar that life is empty without love. Filippo echoes her thoughts and states how much he loves her; she has no equal. His courtiers again sympathize with him and encourage him to seize the moment. Agnese disappears and all leave. Then Agnese reappears, this time singing for Orombello. Mysteriously, she wishes that her heart will guide him to her arms and, as in all good opera plots, the object of her lust makes his entrance. Orombello splutters that he does not know where he is or why he is there. Comforted by Agnese, he begins to relax and agrees that he is deeply in love and, when asked about a letter, shows her the one he is carrying. "Such misfortune!" The letter he is referring to is one of many he has written to Beatrice and not the one that Agnese had sent to him. Agnese's world falls apart, her tenderness turns to vitriol, and the two of them spit out a dramatic aria and leave.
Beatrice enters one of her secret places with her ladies. She is happy, but soon loses her poise and laments how misguided she has been to have married the evil Duke Filippo. As they all go to leave, Filippo sees them in the distance and, believing she is avoiding him, demands that she be brought back. The two of them accuse and rage at each other, with Filippo producing some secret papers stolen from Beatrice's apartment.
In another scene, slightly the worse for wear, Filippo's soldiers discuss his silence and temper. Beatrice enters carrying a portrait of her beloved, deceased husband, Facino. She is bemoaning the fact that everyone has abandoned her when Orombello enters protesting that he has not. Excitedly, he tells her his plans to rally the troops and help her free herself. She crushes him saying, in so many words, that she does not rate his expertise in security matters. Stunned, Orombello protests his love and, even when begged to do so, will not leave her presence; instead, he kneels down in front of her, at which moment Agnese and Filippo enter and accuse the two traitors of having an affair. Everyone now joins in with accusation, counter accusation, attack and defence. The upshot is that Filippo has the pair arrested - to be tried in Court for adultery.
The courtiers learn of the terrible torture that has been applied to Orombello. Then, the Court is summoned and Filippo sets out the case for the prosecution. Beatrice is dragged in, and she protests that the Court has no jurisdiction. Next, Orombello is hauled in and, after desperately seeking forgiveness from Beatrice, proclaims her innocence. Beatrice regains her will to live and something in her speaking touches Filippo's heart. He announces that the sentence should be delayed. The Court overrules him stating that more torture should be applied until the truth is spoken. Again, Filippo changes his mind and, supporting the Court's decision, instructs that, indeed, more torture seems to be necessary to extract the truth. The Court rises.
Filippo and Agnese, full of remorse, are left alone and Agnese, realizing that things have gone much further than she had expected, begs Filippo to drop all the charges; but Filippo, not wishing to look weak, dismisses the idea.
Filippo now goes through several stages of torment and is obviously still deeply in love with Beatrice. Just as he has made up his mind to drop all the charges, with cruel timing, men still loyal to the late condottiere Facino arrive, to invade the castle. As a result, Filippo signs the death warrant now handed to him by Anichino* and tries to justify his actions to the crowd, blaming Beatrice's behaviour.
There is a scene in which we see Beatrice's ladies outside Orombello's cell, while Beatrice prays. The action reaches its finale.