Lambert Strether arrives in England as a representative of his benefactress, Mrs. Newsome, with the express purpose of convincing her son, Chad Newsome, to leave off the carefree life he has been leading in Europe and return to the family business in Woollett, Massachusetts. Strether, convinced at the start of his journey that his ambassadorial mission is a clear-cut one, soon recognizes the ambiguity of the situation through the comments and questions of his newly found "guide" to Europe, Maria Gostrey.
Strether finds Paris filled with "old ghosts," haunting him with memories of his brief honeymoon there many years ago. Soon after arriving, he meets two of Chad's friends, Little Bilham and Miss Barrace. They are full of praise concerning Chad, and Strether becomes further disturbed about the correctness of his mission. When he finally meets Chad, he finds the young man has "been made over." Strether "had been wondering . . . if the boy weren't a Pagan, and he found himself wondering now if he weren't by chance a gentleman."
Strether is introduced into Chad's society at a garden party at the home of Gloriani, the famous sculptor, and it is here that he meets Madame de Vionnet and her daughter, Jeanne. At first, Strether assumes that Chad is involved with the daughter, but a few days later, he learns that it is Madame de Vionnet whom Chad is seeing. Although it is common knowledge among Chad's circle of friends that the two are having an affair, Little Bilham tells Strether the attachment is "virtuous," and as Strether already sees Madame de Vionnet as the ideal woman, he does not question the declaration.
Paris life with its free atmosphere begins to take a firm hold on Strether, and finding himself completely captivated by Madame de Vionnet, he agrees to help her. When Chad surprises him by consenting to return to America immediately, Strether suggests that Paris may well offer more to them than Woollett. Chad leaves the decision up to Strether but reminds him that by delaying their return, Strether is placing in jeopardy his relationship and pending marriage to Mrs. Newsome; Strether willingly accepts the risk.
Mrs. Newsome, made impatient by the lack of results from Strether's mission, sends her daughter, Sarah Pocock, to rescue both Strether and Chad from their Parisian influences. Accompanied by her husband, Jim Pocock, who has been running the family business in Chad's absence, and her sister-in-law, Mamie Pocock, Sarah arrives in Paris and proceeds to exercise her influence on Chad and Strether. Mamie, who was to serve as a lure to attract Chad, finds in Little Bilham a friend and escort.
Strether hopes that Sarah will realize how good the Paris experience has been for Chad, how much Madame de Vionnet has improved him, and relay the knowledge to her mother. Sarah, however, does not waver from her task and issues an ultimatum that the two men are to make their decision while she and her family are on a side trip to Switzerland. During the period of the Pococks' absence, Strether accidentally discovers that Chad and Madame de Vionnet are lovers. Although disturbed by the discovery, he continues to "help" Madame de Vionnet by telling Chad to remain in Paris. When Strether confronts Chad, the young man leaves him with the impression that he will stay in Europe for a time but will eventually return to America. Finally, Strether makes the decision for his own future. He has freed himself from Mrs. Newsome's influence, but he is unable to accept the love Maria Gostrey offers him. He will return to Woollett.