"The Moonlight" is the account of a conversation between Madame Julie Roubere and her elder sister Henriette Letore.
During the exposition, the author tells the reader that Madame Roubere is waiting at her home for her sister, Madame Letore, who has just returned from a trip to Switzerland. Madame Letore has come to Paris to visit alone, her husband having returned to their estate early to take care of some business.
When Madame Letore arrives, the action begins its rise. The sisters embrace, and Madame Roubere is astonished to see that Madame Letore has "two large locks of white hair". When Madame Roubere presses her sister to tell her what is the matter, Madame Letore reveals tearfully that she has a lover. Madame Letore explains that she is fond of her husband, "but he is mature and sensible, and cannot even comprehend the tender vibrations of a woman's heart". Madame Letore herself is a woman of deep feelings and emotions, and she has for quite awhile longed for more passion in their lives. Madame Letore had never meant to deceive her husband, but the final straw came during the time they had just had, traveling together.
In the climax of the story, Madame Letore describes how she had been walking with her husband one morning, and had been overcome by the beauty of the landscape around them. She had exclaimed about the scene, and asked her husband to kiss her, but he had responded with "chilling kindness" that the fact that she liked their environs was no reason for a kiss.
A few nights later, Madame Letore was walking alone by the lake, as the falling action begins. A full moon was showing, and the scene again was sublime, evoking strong emotion in the passionate woman. She sat by the lake and was sobbing "like a crazy woman", and a man appeared, a "young barrister" with whom she and her husband were distantly acquainted. They talked for awhile, and he commented on the sights they had seen on the trip and recited some verses of poetry. Seized with "indescribable emotion" that the young man should so clearly understand what she was feeling, she made love with him.
Madame Roubere consoles her sister as the older woman shrieks in anguish at the turn her life has taken. Gently, she tells her that it is not the man she loves, "but love itself", her "real lover that night" having been the the idea of love as represented by the moonlight.