In the spring, around 1840 in Paris, Jean-François Billembois, a young artisan-carpenter, struggles to find the words to describe to his friend Pancrace the show he saw the night before at the Théâtre des Funambules. Nanette, a sweet young girl, arrives and reminds Billembois that he promised to take her on a walk to Montmorency the following Sunday. But Billembois finds an excuse to break his promise. When Pancrace expresses surprise at this change of heart, Billembois confides that he is madly in love with the beautiful Zéphyrine, an actress at the Théâtre des Funambules, but that he loses his nerve every time he sees her. His friend Pancrace invites him for a drink to cheer him up.
Across from Billembois's shop, in a former mansion, a mysterious foreigner, Mr Tenorio, has taken up residence. Pursued to his home by Anaïs, an old bigot, Tenorio instructs Jasmin, his servant, not to let anyone in.
Old Anaïs accuses Tenorio of using the church to spy on the prettiest women and seduce them with his devilish ways. After a torrent of curses, Anaïs faints in front of the seducer's door. She is rescued by some women and taken to the herbalist, Mrs Fouillechat. In the evening, on the deserted street, Ratapoil appears, a strange figure in uniform who knocks on Tenorio's door and introduces himself as the Commander. But Jasmin does not let him in.
In the back room of the herbalist-witch's shop, Billembois, in despair, complains to Madame Fouillechat that none of the potions or talismans she has given him have made Zéphyrine fall in love with him. She attempts an incantation, but is regularly interrupted by the arrival of other customers. She then asks Billembois to go home and perform a ritual by plucking three hairs, which she will incorporate into a love cake intended for Zéphirine. Meanwhile, Jasmin comes to fetch rejuvenating enemas for his master and a remedy for his fear. When questioned by Mrs Fouillechat, he reveals that Mr Tenorio is in fact the famous Don Juan (albeit aged), who fled Spain to escape the vengeance of the Commander he killed. But the Commander has found Don Juan and has been knocking on their door every night for a week. Jasmin, proud to serve such an illustrious master, makes Mrs Fouillechat swear to keep the secret. When Billembois returns, Mrs Fouillechat changes her plan and sends him to her neighbour Mr Tenorio, so that he can teach him the secret of the secret of making women love him.
Meanwhile, Jasmin complains to Mrs Fouillechat that rejuvenating enemas she preapared for his master are ineffective. Offended, Mrs Fouillechat publicly reveals that Tenorio is none other than the famous Don Juan, now ravaged by old age. Hearing this, the other women, fascinated by the presence of the mythical seducer, bombard Jasmin with questions about him and end up quarrelling. Meanwhile, Zéphyrine manages to break into Tenorio's house.
In the countryside, in Montmorency, near a guinguette, everyone is gathered together, walking, playing music, singing, dancing or flirting. Zéphyrine allows herself to be seduced by Don Juan and regains her confidence. Billembois falls in love with Nanette, whose purity of soul he perceives, and immediately forgets Don Juan's secret. The Commander has followed Don Juan's trail to Montmorency. Don Juan sees Nanette and wants to seduce her, but she rejects him, calling him ‘old’. It is then that Don Juan accepts the Commander's punishment, as his life as a seducer is over. Hailed as the glory of having been Don Juan's last mistress, Zéphyrine is courted by the wealthy Arthur. While Billembois is overjoyed to have found true love, Nanette seems lost in her thoughts, and her last line refers to the colour of Don Juan's eyes.