Lambert Strether, an American from Woollett, Massachusetts, arrives in Chester, England. At the hotel desk, he learns that his old friend Waymarsh has not arrived. Instead, Strether meets a young American lady named Maria Gostrey who claims to know Waymarsh. Quickly becoming friends, Strether explains to Miss Gostrey that he has been sent to Europe by his fiancée, Mrs. Newsome, to fetch her son, Chad. In Woollett, everyone believes that Chad has become romantically involved with an inappropriate woman and refuses to come home so that they remain together. Strether’s mission, as Mrs. Newsome’s trusted ambassador, is to convince Chad to come back to Woollett, where he is needed by the family business. Strether confidently believes that his mission is noble and necessary. If he succeeds, he and Mrs. Newsome will be married. Miss Gostrey suggests that Strether wait to judge Chad until after Strether has seen this lover with his own eyes. Throughout the novel, Strether relies on Miss Gostrey for confidence-boosting advice.

Strether, Waymarsh, and Miss Gostrey head to Paris, where Chad lives. When they arrive, they learn Chad is away. While they wait for Chad’s return, Miss Gostrey leads the two older men on sightseeing trips, and Strether begins to enjoy his time in Europe. Strether visits Chad’s apartment and notices an unfamiliar man lingering on the balcony. Strether makes this man’s acquaintance and learns that he goes by the name “little Bilham.” A friend of Chad’s, Bilham takes care of Chad’s house when Chad is traveling. Miss Gostrey suggests to Strether that Bilham is operating under Chad’s “instructions.” Regardless, Strether finds Bilham charming and invites the young man to the opera. Bilham, however, does not show. Instead, a stranger enters the box. Strether realizes that this new arrival is Chad Newsome. Chad has returned from his travels quite a changed man.

After the opera, Strether tells Chad why he has come to Paris. However, as he speaks, Strether finds himself less certain of his stance. Chad, once callow and juvenile, now seems confident and restrained. His new personality impresses Strether, who wonders what—or who—has caused Chad’s transformation. Chad asks Strether to stay and meet his close friends, a mother and a daughter, who are arriving in a few days time. Strether, wondering if one of these women has been the impetus for Chad’s improvement, and assuming the daughter to be Chad’s lover, agrees to stay. Meanwhile, Bilham convinces Strether that Chad has a “virtuous attachment”—and that Chad’s relationship with the mysterious woman is innocent. Strether eventually meets the women, Madame de Vionnet and her daughter, Jeanne, at a high society party, but he does not see them long enough to cement an impression. After the brief introduction to Madame de Vionnet, Strether finds himself alone with little Bilham. Strether takes the opportunity to offer Bilham some sage advice: live all you can before it is too late. This advice exposes Strether’s own change since coming to Europe. In Paris, he feels renewed, young again, doubly alive.

Over time, Strether comes to think that Madame de Vionnet has been the good influence on Chad. Strether eventually learns that Maria Gostrey and Marie de Vionnet went to school together as young women but have not seen each other for many years. When Chad arranges for the marriage of Jeanne to another man, Strether realizes that Madame de Vionnet is Chad’s reason for staying in Paris. Strether surprises himself by promising Madame de Vionnet that he will try and keep Chad near her. As if aware of this promise, Mrs. Newsome writes Strether and demands that he end his tenure as her ambassador. Her ultimatum: bring Chad home or return to Massachusetts by the next ship. Even though his refusal to return home puts his marriage to Mrs. Newsome in jeopardy, Strether longs to stay in Europe. In a complete reversal, Chad declares that he is ready to return to Woollett, but Strether begs him to stay in Paris. Almost immediately, Mrs. Newsome sends a new shipment of ambassadors: her daughter, Sarah Pocock, along with her husband, Jim, and Jim’s sister, Mamie, who Mrs. Newsome hopes will marry Chad. Strether worries that Sarah will contradict his reports on Chad’s progress and on Madame de Vionnet’s worthiness to Mrs. Newsome.

Initially, Strether believes that Sarah is as charmed by Chad’s Parisian life as Strether was. But soon, Sarah meets Strether face to face and reiterates Mrs. Newsome’s negative views of the situation. She demands Strether’s complete compliance in convincing Chad to go home to Woollett as soon as she, Waymarsh, Jim, and Mamie return from a leisure trip to Switzerland. Waymarsh and Sarah are acutely disappointed in Strether. While the others are away, Strether takes a day-trip himself to the French countryside. There, he coincidentally spots Chad and Madame de Vionnet riding together on a small boat. All at once, he understands: their relationship is unmistakably intimate and obviously lacking in virtue. But, even though Chad and Madame de Vionnet were deceitful, Strether still feels that their involvement has improved Chad as a person.

Afterward, when Strether visits Madame de Vionnet, she seems defeated and convinced that Chad will return to the United States. She insists that she has wanted Strether all along, to which Strether replies that she has had him. Nevertheless, Strether suggests that he can still help her keep Chad. However, after Chad stays away from them both for an entire week, Strether’s resolve begins to erode. When Strether finally confronts Chad, the young man speaks only of plans for improving the family business in Woollett. His desire to return home is unambiguous. Even though Strether thinks that Chad would be more fulfilled in Europe with Madame de Vionnet, he cannot convince Chad to stay. Instead, Strether goes to visit Miss Gostrey, at which point her long-gestating love for him is made obvious. She makes what amounts to a marriage proposal to Strether, but he feels he cannot accept it. Even as he realizes that, compared to Europe, his life in Woollett will be bland, Strether chooses to return to the United States.