In 1868, Christopher Newman, an American businessman, visits Europe on a Grand Tour. Having worked for a living since age ten (interrupted by service in the Union Army during the American Civil War), he has made a large fortune and retired in his thirties, and is now looking to settle down and get married.
At the Louvre in Paris, he watches a painter named Noémie; he offers to buy the copy she is making and meets her father, M. Nioche. About the same time a mutual friend introduces Newman to Claire de Cintré, a young widow. Newman hires M. Nioche to teach him French and the two become friendly; Newman, learning that M. Nioche worries about his daughter's future since he is poor, says that he will buy enough paintings from Noémie to give her a respectable dowry. Meeting Newman at the Louvre the next day, though, Noémie frankly tells him that she has no talent and her paintings are worthless. She scorns the men she could marry even with a dowry, and hints that she would prefer a more exciting life. Newman either doesn't understand the hint or ignores it, and he leaves her to her work. He pays a visit to the Bellegarde estate, where he meets Claire's two brothers: the cheerful Valentin and the aloof Marquis de Bellegarde, who coldly rebuffs him.
Newman and Valentin become good friends, and eventually he tells Valentin that he wishes to marry Claire; Valentin tells Newman that he has his support but he will find it hard going against the class prejudices of the Marquis and his mother, royalist supporters of the Bourbons. Newman proposes to Claire; after hesitating, because her first husband was abusive, Claire says she will consider it.
Newman and Valentin visit the Louvre and find Noémie at work in a gallery. Noémie tells Newman she has finished none of the work she was to do for him, and in irritation she slashes a large red cross over her painting, obliterating it. Afterwards Valentin tells Newman that Noémie is certain to become the mistress of some rich man, though Newman objects that her respectable father would never allow it.
Claire's mother tells Newman his "commercial" background makes him unfit, but when he tells her how rich he is, she reluctantly agrees not to oppose him. After some months Claire agrees to marry him, and the Bellegardes have a party in honor of the engagement. Newman hears town gossip that Noémie has become a courtesan; he goes to see M. Nioche, and finds him drinking in misery.
While Newman is occupied with arranging the upcoming wedding, Valentin becomes involved with Noémie. At a theater he exchanges insults with a rival and they agree to a duel; he leaves for Switzerland to fight the duel despite Newman's exasperated attempt to talk him out of it. The next day the Bellegardes tell Newman that the engagement is off, and Claire has been ordered to leave for the family's estate in Poitiers. He hardly has time to digest this when he gets a telegram telling him that Valentin, wounded in his duel, is dying. Newman hurries to Geneva where he sits by Valentin's deathbed. Valentin, ashamed of his family's behavior, tells Newman that the Bellegardes have a terrible secret, which he can discover in Poitiers and use against them. Newman goes to Poitiers to attend Valentin's funeral, and afterwards urges Claire to disobey her family and marry him. Claire cannot stand against her mother and intends to become a Carmelite nun.
Newman discovers the family secret: Claire's mother indirectly murdered the old Marquis, Claire's father, by throwing out his medicine during his illness and keeping doctors away from him until he died. At first, he plans to use the secret against the family but soon decides that that would be beneath him, and he leaves Paris. Walking in Hyde Park in London he sees Noémie, now a successful woman of fashion, flirting with wealthy men as M. Nioche sits forlornly nearby, and he walks away from them in disgust. After restlessly traveling to New York and San Francisco, he hears that Claire has become a nun; he returns to Paris to look at the walls of the convent from the street and then leaves Paris for good, resolving never to think of the Bellegarde family again.
