Valcour has been hopelessly in love with Léontine, a beautiful young widow, for years. With the help of his trusted accomplice Ophémon, Valcour has been showering Léontine with a stream of gifts, flowers, and love letters. He has done so anonymously; Léontine had been betrayed by her late husband, and Valcour is certain that she has no interest now in pursuing romance with anyone. To help cover up his secret, Valcour has told Léontine that his heart is closed to love too. Valcour tells Ophémon that he has lost all hope of winning her affections, while Ophémon responds that it is time for Valcour to reveal his secret.
Léontine asks Valcour to settle a dispute she is having with her friend Dorothée. The Anonymous Lover has sent Léontine a bouquet of flowers along with a letter asking her to carry it at a wedding later that day; if she doesn’t, he will take it as a sign that he should stop sending gifts and leave her alone. Dorothée thinks that it would do no harm for Léontine to accept the flowers. Léontine doesn't want to hurt her secret admirer’s feelings but doesn’t want to lead him on either. Valcour convinces Léontine to carry the bouquet.
Jeannette and Colin, the young couple about to be married, arrive and express their thanks to Léontine for making their wedding possible. They hope that everyone might be as filled with love and joy as they are.
When Léontine and Dorothée are alone, Léontine recalls her heartache when her late husband's love grew cold during their brief time together, and how Valcour’s friendship helped her get through it.
Dorothée, who has been separated from the man she loves, reflects on the unhappiness that love has brought her, while dreaming for a happy reunion in the future.
Valcour tells Léontine that the Anonymous Lover might very well be watching them nearby at that very moment. He convinces her to call out to her mysterious suitor and ask him to reveal himself. When she does so, Valcour overdramatically—and clumsily—answers that he himself is the tormented Anonymous Lover...a declaration that makes Dorothée laugh at Valcour’s “joke” but leaves Léontine strangely confused. Valcour and Ophémon quickly try to convince Léontine that it was all in jest, while Jeannette and Colin are sympathetic to Léontine’s obvious emotional distress.
Alone, Léontine laments that Valcour is unlikely to understand her feelings, but she wishes that she could unburden her heart to him, since he has been such a trusted friend. Ophémon arrives with news that he has spoken to the Anonymous Lover, and Léontine presses him for details while Ophémon resists, prolonging the conversation while gleefully watching her attempt to conceal her eagerness. Finally, Ophémon describes how the Anonymous Lover worries that it would be impossible for Léontine to love him, but that he wishes to reveal his identity anyway. Léontine agrees to the meeting, but her obvious reluctance convinces Ophémon that she is in love with someone else: Valcour.
As she awaits the arrival of her secret admirer, Léontine reflects that her heart is now full of passion, something she never expected. When Valcour shows up, explaining that he’s just there to be her supportive friend, Léontine desperately tries to get him to leave before the Anonymous Lover’s arrival. As Ophémon listens outside the door, Léontine and Valcour finally confess their love for each other as Valcour reveals that he himself is the Anonymous Lover.
Léontine tells an astonished Dorothée everything, and they all join Jeannette and Colin for what is now a double wedding. The two couples celebrate their happiness with songs and dances.