Terrace of Grimani’s Palazzo in Venice. A masquerade takes place at night. The terrace facing the Giudecca canal is full of merry young people. They are Liverotto, Gazella, Vitelozzo, Orsini and Gennaro. This is their last day in Venice, for tomorrow must go to the court of Duke Alfonso. Gubetta, Lucrezia’s servant, pretending to be a Spanish nobleman, enters the conversation. As soon as he mentions the name of Lucrezia Borgia, curses are heard, and Orsini talks about a terrible prophecy. Once was saved from death on the battlefield by Gennaro. Later a giant old man in black appeared to Orsini predicting that he and Gennaro would die on the same day from the hands of Lucrezia (Nella fatal di Rimini e memorabil guerra…). The fun fades out. Gennaro falls asleep on the terrace, while the others leave.
A gondola approaches. It carries a masked woman who comes out, bending over Gennaro. This is Lucrezia Borgia. With love, she looks at the sleeping young man. This is her son, who was brought up secretly, never seeing his mother (Com’? bello! Quale incanto…). Duke Alfonso and his servant Rustighello watch her unnoticed. Recognizing his wife, the duke disappears. Gennaro wakes up. Fascinated by the beautiful stranger, he confides his secret to her. He grew up in a poor Neapolitan fisherman’s family, but one day an unknown man gave him a weapon, a horse and a letter from his mother. The letter said that she was a victim of her ruthless relatives belonging to a famous family who had killed his father. Gennaro dreams of meeting her (Di pescatore ignobile esser figliuol credei…). Gennaro’s friends return. Orsini recognizes Lucrezia Borgia. Everyone starts blaming the Duchess for murders, saying the names of the deceased ones. Lucrezia is in despair. The young men and Gennaro leave.
Scene 1. City square in Ferrara
Duke Alfonso and Rustighello watch a little house where Gennaro has fun with friends every night. Convinced that his wife has been unfaithful to him, the duke swears to wash the disgrace away with the blood of Gennaro (Vieni: la mia vendetta…). The Duke enters the palace. Gennaro and his friends come out of the house. They arrange to meet at the feast of duchess Negroni. Liverotto makes fun of Gennaro, asking if Lucrezia has bewitched him. Gennaro replies that he hates her and knocks the first letter of her surname from the Borgia emblem, leaving the sparkling word orgia. The incident is watched by Rustighello and Astolfo, Lucrezia’s guard, who has been instructed to bring the young man to the duchess. Rustighello calls on the guards forcing Astolfo to retreat.
Scene 2. The palace of Duke Alfonso
Rustighello reports to the duke that Gennaro has been arrested. Lucrezia appears complaining of the insults she has suffered in Ferrara and demanding that the man who damaged the Borgia emblem should be found and executed. Don Alfonso reports that the duchess’s will is already satisfied, and orders the arrested man to be brought. Lucrezia is horrified to see Gennaro. Left alone with her husband, Lucrezia begs the duke for mercy, but he refuses, considering the young man his wife’s lover. When the guards bring Gennaro in again, the duke pretends to forgive him and proposes to drain the goblet for friendship, while ordering Lucrezia to fill it with poison (Guai se ti sfugge un moto…). Gennaro drinks the goblet and the duke leaves. Lucrezia says to Gennaro that there was poison in the wine, offering him a bottle with an antidote (Bevi e fuggi … te’n prego, o Gennaro…). The young man refuses to accept it, suspecting a lie, for Borgia is well known as a poisoner. Lucrezia, as mother, begs the young man to give in and Gennaro drinks the antidote.
Scene 1. A back street near Gennaro’s house
Assassins, led by Rustighello, gather in the courtyard Gennaro’s house at night. Orsini comes to invite his friend to the farewell ball at Princess Negroni’s palace. Gennaro says that he is leaving Ferrara, as he is in danger here. Orsini calls Gennaro a coward persuading him to stay. Gennaro agrees. After swearing eternal friendship, the young men go to the ball. Rustighello, who heard the conversation, hurries to the palace to report everything to the duke.
Scene 2. A hall in Negroni Palace
Orsini and Gennaro join the merry assembly. Gubetta soldered the Venetians, provoking a quarrel with Orsini so that everyone except Gennaro’s friends leave the feast. Orsini sings a drinking song (Il segreto per esser felici…). Suddenly, the lamps go out and Lucrezia appears. Coming to take revenge on the Venetians for the insults, she informs them that the wine was poisoned and all of them should be dead in a few minutes. Suddenly Lucrezia notices Gennaro among the guests. She orders the guards to take his friends away and begs Gennaro to take the remaning antidote in the bottle. Gennaro wants to share it with his friends, but Lucrezia says that there is barely enough medicine for him alone. Gennaro decides to die with his friends and stabs Lucrezia to avenge their death.
Lucrezia reveals to her son the secret of his origin and begs him to forgive her. Throwing the dagger away, Gennaro dies on his mother’s chest. The duke enters with his retinue to find out that he killed not the rival, but the son of his wife. Lucrezia falls unconscious.