The carnaval on Monday
The Pope's treasurer Balducci is displeased that the jeweller and sculptor Benvenuto Cellini, and not his rival Fieramosca, has been asked by the Pope to create a statue of Perseus; Balducci plans to have his daughter Teresa marry Fieramosca. Teresa receives a love letter from Cellini; soon he will come to see her when her father is absent. The lovers plan to elope. Unseen, Fieramosca intrudes and listens to their converstion. Upon his return, Balducci discovers his daughter's unexpected visitor, and Cellini manages to conceal himself. The father is enraged and plans to punish his daughter's suitor.
The carnaval on Tuesday, evening
Cellini is feasting with his friends. There is no more money for wine, but the imposing account from the shop must be paid. Fortunately Ascanio, Cellini's devoted pupil, arrives and brings his advance fee for his work on the statue of Perseus. Truth be known, the advance fee is miserable. The friends resolve to ridicule Balducci. At the same time, Fieramosca, who has summoned the bandit Pompeo for assistance, plans to interfere with Cellini's plans to abduct Teresa. The guests assemble for the dinner party. After a theatrical performance Cellini wishes to depart with Teresa, but Pompeo – dressed exactly the same – appears before the girl. A dispute arises between the rivals and Cellini kills Pompeo. The murderer manages to flee, but Fieramosca is arrested.
Scene 1. First day of Lent, morning
Teresa and Ascanio pray for Cellini's salvation. To their great joy he is alive and well, and suggests that Teresa flee without delay. The couple, so much in love, are barred by Balducci and Fieramosca who has been released. The father damands that Teresa marries Fieranosca. Cellini threatens to kill his rival. The quarrel is interrupted by the arrival of the Pope, who has come to see Perseus. Seeing that the sculpture is not ready, he declares that he will give the commission to another sculptor. Cellini despairs, and the Pope gives him a deadline together with an ultimatum: if the statue is completed by evening the sculptor will be pardoned of all his sins and he may marry Teresa. If not, he will be executed.
Scene 2. Evening the same day
Ascanio impatiently awaits his master's new creation. But Cellini despairs: he is met on his way with innumerable obstacles. Fieramosca appears and calls him to a duel. Teresa tries to stop her beloved taking part in the duel, but Cellini assures her that there is no danger and he departs. His assistants enter the atelier. They are furious that they have not been paid for ages. But, when Fieramosca tries to bribe them and make them work for him, their respect for their master takes the upperhand over their displeasure. They are well-prepared to take care of the schemer when Cellini returns. Almost frightened to death, Fieramosca agrees to assist his former rival. The Pope and Balducci arrive. The statue begins to be cast. But during the casting process it transpires that there is not enough metal. Cellini is ready to dedicate everything including himself to create this masterpiece. At last Perseus is ready and all present praise the great craftsman.