Act one

Paris, around 1860, in Magda's salon

The poet Prunier provokes the guests with his ideas about love. Only Magda believes in romantic love. She tells a story that touches even her unromantic benefactor, the banker Rambaldo. He gives Magda an expensive pearl necklace. Prunier reflects on the contradiction between Magda's belief in true love and her work as a courtesan.

Magda's maid Lisette announces a young man who wants to see Rambaldo. It is Ruggero, the son of one of Rambaldo's old friends. Prunier wants to read Magda's hand. His prophecies are ambiguous. Her life could be as beautiful as that of a swallow in the sun. But there are also dark clouds looming. Young Ruggero wants to know how to spend his first night in Paris. Lisette suggests the Bullier dance club. The guests leave and Magda is left alone. She decides to go to Bullier herself, disguised as a labourer.Lisette and Prunier meet in secret. The writer confesses his love for the simple girl. Nonetheless he is forcing her to completely change her wardrobe by "borrowing" clothes from her mistress before he wants to appear in public with her.

Act two

At the Bullier dance club

Young men and women seduce each other. Ruggero sits shyly at a small table. Magda, in disguise, enters. She is immediately besieged by students who ask her to dance. Magda rejects them by pretending she has a date with Ruggero. The two dance, talk and kiss. Lisette and Prunier enter the pub and mingle with the dancers. Lisette recognises Magda. They play a game of deception with each other. This is followed by a love quartet between the two couples. Ruggero is fascinated by this mysterious woman. / Magda is torn between love and remorse. / Lisette adores the poet. / Prunier creates his own muse. Then the jealous Rambaldo enters the dance hall. Prunier tries to save the situation.

Magda declares that everything is over between her and Rambaldo. A voice announces the dawn and warns: Don't trust in love! Magda tells Ruggero about her fears for their future. Ruggero takes her in his arms and they leave together.

Act three

On the Côte d'Azur

A few months later, Magda and Ruggero are staying in a quiet hotel by the sea. Ruggero is overwhelmed by his happiness. Magda hides her doubts. Ruggero writes to his parents and asks them for money and their consent to marry Magda. Magda realises that she can no longer hide her past. Prunier and Lisette have discovered the hotel. Lisette has had a disastrous debut as a theatre actress and wants to work for Magda again as a maid. On behalf of Rambaldo, Prunier tells Magda that she can no longer live a lie. She must return to her old life in Paris, just as Lisette returns to her life as a maid. Ruggero storms in with a letter from his mother. She agrees to the marriage as long as the bride is a decent person. Magda cannot accept his proposal. She reveals her past to him. Ruggero doesn't want to hear it. He is overcome with grief. Magda bids farewell to her one true love and departs for an unknown destination.