ACT I
An elegant salon in Magda's house in Paris
Magda, the mistress of the rich banker Rambaldo, is serving tea for her guests. Yvette. Bianca and Suzy laugh at the poet Prunier, who is holding forth about his theories of love. When he speaks approvingly of sentimental love, Magda's maid Lisette tells him he does not know what he is talking about. Prunier is offended and Magda dismisses Lisette. Prunier insists that everyone is prey to romantic love; to prove his point he starts the story of his latest heroine, Doretta, who spurned a king's ransom for love. When Prunier admits he does not know how to finish his song, Magda improvises a second verse, telling how Doretta lost her heart to a student. All the guests are charmed by her performance, Rambaldo so much so that he presents her with a pearl necklace.
Lisette announces a stranger who has been repeatedly inquiring for Rambaldo. Rambaldo asks Magda for permission to receive the son of an old school friend. Magda describes a youthful escapade she once had at Bullier's, where she met a charming student with whom she had a delightful flirtation. The girls suggest Magda's story to Prunier as the subject for one of his songs but he retorts that he prefers perverse heroines like Berenice and Salome.
Prunier takes the girls on one side to demonstrate his skills at palmistry. Lisette shows in the visitor. Ruggero Lastouc and Rambaldo reads a letter he has brought from his father. Meanwhile, Prunier, reading Magda's palm, tells her she is like a swallow, eager to migrate towards the sun and true love. Ruggero, who has never been to Paris before, expresses his excitement at being in the city and Rambaldo calls on the others to suggest a suitable place of entertainment for the evening. Prunier advises Ruggero to go to bed. Lisette interrupts and recommends Bullier's.
After Ruggero has left Magda chides tbe others for making fun of Ruggero. Her guests leave and Magda tells Lisette she will be staying in; but after a moment's reflection she impulsively decides to go to Bullier's and hurries off to change. Soon Lisette comes tiptoeing in, dressed up to go out; Prunier is waiting for her. He tells her he dislikes her hat, which is Magda's, and Lisette goes to change it. They steal off together.
ACT II
At Bullier's
A pleasure-seeking crowd of students, artists and flower girls fills the place with animated chatter. Suddenly their attention is caught by Magda, who enters shyly. Some young men gather round her but she escapes them and hurries over to the table where Ruggero is sitting alone. She apologizes and explains that she will leave him as soon as the young men are no longer watching. Ruggero, who does not recognize her, gently urges her to stay and they chat amicably; she reminds him of the girls back home. Ruggero persuades her to dance. Soon the dance floor is crowded.
Prunier and Lisette appear. They are having an argument because of Prunier's insistence on trying to turn her into a lady, and Lisette leaves him, angry. Magda and Ruggero return to their table. When Ruggero orders two beers, Magda is filled with memories of her earlier escapade. Ruggero asks her name and she replies 'Paulette' and writes it on the table; Ruggero signs his name beneath it. Magda teases him about his probable love affairs, to which he replies that if he were to fall in love it would be for ever. Their mutual attraction grows in intensity and soon they arc kissing.
Lisette turns up again, now convinced that 'Paulette' is her mistress, a deduction that Prunier pretends not to believe. To prove his point he takes Lisette and introduces her, but Magda tries to preserve her disguise. The two couples sit at the same table and soon are toasting their love. Suddenly Prunier catches sight of Rambaldo. Upset, Magda begs Ruggero to leave her alone for a few minutes. When Rambaldo demands an explanation, she answers that he has seen it for himself and she has nothing to add. Rambaldo asks her to leave, but Magda refuses, swearing she loves Ruggero. Rambaldo bows. Magda is left alone. Ruggero joins her and the two stroll away.
ACT III
A hotel on the Riviera
For some months Magda and Ruggero have been leading an idyllic existence; they discuss their first meeting and their present happiness. Ruggero tells Magda that he has written to his mother, not just for money to pay their many debts but also for consent to his marriage to Magda. Knowing that her concealed past will make her unacceptable to Ruggero's parents, Magda suffers as she listens to his description of wedded bliss in the bosom of his family. Ruggero goes off leaving Magda torn between her desire to tell Ruggero the truth and her wish to avoid hurting him.
Lisette and Prunier appear. Lisette is distraught because Prunier has tried to make her a music hall singer and her performance in Nice the before was a fiasco; the whistling of the audience still rings in her ears. When Magda sees them she welcomes Lisette back as her maid. Prunier gives Magda an oblique message from Rambaldo, who will be happy to take her back on any terms. Lisette goes off to resume her duties and Prunier leaves after finding out what time Lisette will be free that evening.
Ruggero returns with his mother's letter, which he shows Magda. She reads about Ruggero's mother's hopes that her son has chosen a virtuous woman who will be a worthy mother of his children. Unable to keep silent any longer, Magda reveals her past, telling the stunned Ruggero that she can never be his wife. Ruggero protests and begs her not to abandon him. She tells him he must return home. Ruggero collapses while the heartbroken Magda leaves him.
