ACT I. Pope Clement VII wants to be known as a generous art benefactor and collector. The prelate has commissioned Benvenuto Cellini, a hot blooded but brilliant Florentine sculptor and goldsmith, to create a statue of Perseus killing the Medusa. The Pope's treasurer, Balducci, mistrusts Cellini, preferring a local (and mediocre) sculptor, Fieramosca, to whom he has promised his beautiful daughter, Teresa, who in turn is in love with Cellini.
It is carnival time. Teresa watches the masked revelers from her window and hopes to catch a glimpse of her lover, Cellini. Though Balducci is suspicious, he hurries off to see the Pope. He feels threatened by the rebellious "bandit of genius" Cellini, and intends to persuade the Pope to support the safe and submissive Fieramosca instead of him.
Teresa is a dutiful daughter and is worried to suddenly discover Cellini in the house. Between fear and hope, the two sing of their love for each other and their disdain for Fieramosca, little realizing that the latter has also stolen into the house and is eavesdropping on them. Cellini reveals his plan to elope with Teresa to Florence: Balducci is to attend the theater and while he watches the play, Cellini and his apprentice Ascanio, disguised as monks, will abduct Teresa. Though she fears angering her father, Teresa agrees to the plan. The spying Fieramosca overhears and vows to thwart them.
When Balducci returns unexpectedly Cellini escapes but Fieramosca is discovered hiding in the house. Balducci refuses to hear any excuses and calls the neighbors and servants to punish the intruder. A group of women in their nightgowns unleash their fury on Fieramosca, who, for a moment feels like "Orpheus pursued by the Bacchantes."
Cellini muses on this new feeling: for the first time love has supplanted his desire for art and fame.
At the inn, Cellini's friends and fellow metalworkers drink to the glory of their "divine art" until the Innkeeper comes to settle the tab. Just in time, Ascanio arrives with Cellini's commission for the bronze statue of Perseus which must be cast the next day. Thanks to Balducci the funding is much smaller than expected. Infuriated, Cellini instructs the actors to mock Balducci in the commedia he will soon be attending.
Fieramosca vows to reveal to Balducci Cellini's plan to elope with Teresa, but his friend Pompeo advises him to steal the plan instead. They will disguise themselves as monks and abduct the girl.
Balducci and his daughter arrive at the Carnival, where the play will be presented. Although Balducci hates the theater, he is accompanying his daughter as a favor. The players exhort the Roman spectators to watch their commedia, which, at Cellini's instigation, pokes fun at the Papal treasurer. Feeling sorry for her father, Teresa begs him to leave, but Balducci angrily insists on staying to the end. When Balducci can take no more he attacks the players who are mocking him and in the hubbub both sets of false monks fall upon Teresa, then upon each other. Cellini stabs Pompeo and is arrested. Just when all appears lost, a cannon is fired from the Castel Sant'Angelo, signaling curfew and the end of Mardi Gras. All candles and lights are extinguished, and in the darkness and confusion Cellini escapes and Fieramosca is mistakenly arrested and accused of murder.
ACT II. Early the next morning Teresa and Ascanio look for Cellini in his atelier, hoping in vain to find him hiding there. Hearing the chant of monks in procession, they feel inspired to pray for Cellini's safe return. Cellini enters and recounts his tormented night and miraculous escape. Teresa and Cellini see their reunion as a sign of God's blessings and vow never to be parted and to flee together to Florence. Ascanio tries in vain to remind Cellini of his duty to cast the statue of Perseus. "To hell with the statue!" says the sculptor. The lovers sing of the happiness that awaits them, free, in the mountains, away from the materialistic world, but their flight is cut short by Balducci, who bursts in with Fieramosca, ordering him to take his "wife" home. At this point the Pope unexpectedly appears. His mind is on the statue, and finding out it is not ready yet, furiously decides to let somebody else cast it. Cellini is outraged and would rather die than let another-may he be Michelangelo himself-finish his work. The Pope orders the guards to arrest him, but Cellini grabs a hammer and threatens to destroy the mold of the statue if the Pope does not grant his wishes. The Pope, who cares more about his statue than anything, has to give in and grants Cellini an unconditional pardon, as well as Teresa's hand and time to cast Perseus. But, having the sculptor off guard with all these promises, he delivers the surprising condition attached to them: the statue has to be ready in one hour, or else he will be hanged.
Cellini has set up an immense foundry. The tension is at its peak. To fight his fears, Ascanio laughs and sings. Cellini feels the eyes of Rome upon him and wishes he could live the simple life of a shepherd, free from the worries and pressure that an artist has to go through to please his benefactors. But there is not much time for daydreaming. Ascanio and Cellini are rallying workmen to prepare bronze for the casting. Accompanied by swordsmen, Fieramosca enters. Under the pretext of asking satisfaction from Cellini and provoking him to a duel outside, he plans to take advantage of the sculptor's absence to bribe the workers against their master. The moment is favorable because the workers, dissatisfied with their low wages, just went on strike. But Teresa turns the plan to Cellini's advantage and the workers turn on Fieramosca, thinking that he killed their master. To their surprise, Cellini returns and orders the workmen to dress Fieramosca in an apron and put him to work. Fieramosca happily submits to this punishment.
The Pope and Balducci come to watch the casting; both are skeptical about the success. Fieramosca bursts in asking for more metal saying there is not enough to complete the job. In a creative inspiration, Cellini orders his apprentices to sacrifice all his previous works-statues and jewels-in order to save the casting. The workers redouble their efforts, but the overloaded crucible explodes. For a moment, it seems all is lost. Then the bronze begins to flow.
The casting is successful. Fieramosca is overcome by emotion and embraces his rival, Balducci suddenly changes his mind and willingly hands Teresa to Cellini, and the Pope pardons Cellini. Art has triumphed and everyone sings its praise.