Act One:

During a dinner party at the Parisian home of Magda, a courtesan, conversation turns to the subject of love. A poet in the group, Prunier, warns the women that everyone in town is being bitten by the love bug. But, he asserts that romance is just a fad- an epidemic of madness. The women laugh off the advice except for Magda, who is more serious about love. Then Prunier mentions Doretta, the heroine of his latest poem, who has been bitten by the bug. Encouraged to share the song, Prunier sings that Doretta dreams that she is pursued by a king who offers her riches if she will just love him. Prunier trails off claiming he doesn't know how the lyric should continue. Magda says Doretta moves on and discovers the meaning of passion after one intense kiss from a young student. Magda's lover, Rambaldo, compares romance to weakness and says it's something he's armed himself against. He then surprises Magda with an expensive necklace. Though she accepts the gift, she is not impressed with it. Rambaldo leaves the room to greet a visitor at the door. Magda's friends remark how fulfilling her life must be with the Rambaldo's generosity. Magda quickly retorts that money doesn't buy happiness. She recalls an intensely romantic evening spent with a young man at a night spot called Bullier's several years earlier. She doesn't share much of the details, to the dismay of her friends. Rambaldo returns to the room, accompanied by the guest, who is the son of a childhood friend. As the men talk, Prunier reads Magda's palms. He sees he sees that she is like a swallow, destined to fly toward love. Prunier then reads that destiny will present two faces. But, no one knows if it's a smile or anguish. With that, Prunier moves on to the other women. The conversation then turns to Ruggero, who is asking where he should go that night to meet people, as he's new to Paris. The group offers a quick list of suggestions and decide he should head to Bullier's. With that, the group disbands, everyone leaving the house. Magda dashes off to change her outfit as she's secretly decided to go to Bullier's, as well. Prunier returns to meet Magda's maid, Lisette, with whom he is having an affair.

Act Two:

Magda enters Bullier's and stands out from the crowd because she is dressed as a working class woman.

She catches the eye of Ruggero, who invites her to sit with him. They hit it off, though he doesn't realize he was just inside her house. As the evening proceeds, Magda begins to feel young again. She is reliving the memory she spoke of earlier in the evening. Prunier and Lisette enter Bullier's and eventually spot Magda and Ruggero, though Prunier pretends she is someone who just looks like Magda. Lisette is convinced otherwise. The couple walks over to greet Ruggero and the three of them play along that Lisette is just drunk and imagining the woman is her employer. However, the game ends when Rambaldo walks in. Prunier tells Magda to leave but she insists on owning up to what she's doing. When Rambaldo asks her to go home with him, Magda ends their relationship by proclaiming her love for Ruggero.

Act Three:

As Magda and Ruggero are enjoying a quiet morning at their seaside hotel in southern France, he confesses that he's asked his parents for money and their blessing to marry Magda. She is elated but wonders if she should tell him of her life as a courtesan. Meanwhile, Prunier and Lisette arrive in the same town, having failed at turning Lisette into a singer. She speaks of returning to her life as a maid. Prunier spots the hotel and they ask for Magda. He explains that Lisette wants her job back. When Lisette leaves the room, Prunier reveals his real reason for coming there. He encourages Magda to return to Paris, to her real life and Rambaldo. Prunier says Magda has to abandon this illusion with Ruggero. He tells Magda that Ruggero's mother can only bless their union if Magda is good, mild, pure and possesses all the virtues. With that, the illusion fades and Magda realizes she can never stay with Ruggero. She can be a lover but never a wife. Ruggero begs her to remain as he can't live without her, but Magda, older and wiser, leaves him knowing that her past would prevent them from ever being happy.