The protagonist of the opera is a famous Goldsmith, Rene de Cardillac. For some time before the start of the action, all the people who have purchased his creations have been attacked, robbed of their purchase, and murdered. The criminals have not been found and Paris is terrified.

Scene 1: Another murder has been committed. The Captain of the Guard placates the angry citizens by announcing that the King has set up a torture chamber to wrest the truth from suspects. Cardillac passes by on the way to his workshop and is treated with great respect. The Lady wonders why, so the Cavalier tells her about the beauty of Cardillac’s work and the dangers surrounding it. She is aroused, and promises him a night of love if he will bring her the most beautiful of the Goldsmith’s creations.

Scene 2: The Lady waits for the Cavalier. He arrives with a belt made by Cardillac. While the two are in each other’s arms a masked figure appears, grabs the belt and stabs the Cavalier.

Cardillac leaves his workshop to examine the Gold Merchant’s stock, entrusting the workshop to his Daughter. She anxiously awaits the arrival of her lover, the Officer, with whom she has planned to elope. But when he arrives she realizes that she cannot bring herself to abandon her father. The Officer leaves, vowing that he will make Cardillac release his Daughter. Cardillac returns. The King, accompanied by courtiers, pays him an unexpected visit, but the Goldsmith manages to dissuade him from making a purchase. Cardillac declares that he would have been forced to kill even the King if he had insisted on buying one of his creations. The Officer then returns to demand the Goldsmith’s masterwork – meaning, Cardillac is relieved to discover, the Daughter. The Goldsmith explains that he draws all his strength from his creations, the only things which can remain faithful to him. The Officer sees his chance of breaking the Daughter’s ties to Cardillac, snatches a chain, tosses some coins on the table and leaves. The absence of the chain disturbs Cardillac more and more. No longer able to control his obsession, he rushes out.

The Officer, wearing the chain around his neck, is walking along a street. Cardillac appears and tries to stab him. The Gold Merchant calls for the police and demands that Cardillac be arrested as the killer for whom they have been looking. The Officer, however, accuses the Gold Merchant of trying to shield the real killer. The citizens now cheer Cardillac and lead him off in triumph. Cardillac cannot endure the jubilant mood of the crowd. He tries to defend the unknown killer, soon admitting that he knows him as well as he knows himself. The citizens demand that he tell them the killer’s name and, when the Goldsmith refuses, threaten to destroy his workshop. Only then does Cardillac reveal that he is the very man they have been hunting. Enraged, the crowd lynches him. The Officer, who had unsuccessfully tried to stop them, berates the citizens for taking justice into their own hands, and claims that a sacred madness drove Cardillac to his deeds.