The story starts with the main character, Lambert Strether, an American from Woollett, Massachusetts, travelling to England. He arrives in Chester and goes to the hotel he has a meeting arranged with his old and faithful friend named Waymarsh. He asks if Waymarsh is already here, but learns from the hotel desk that he haven’t arrived yet. Lambert checks in and decides to wait for him. He meets a lady, who is also from America. Her name is Maria Gostrey and she claims that she knows Waymarsh (though Lambert has no idea who she is). Maria is an overly sweet person and Lambert quickly befriends her. While chatting, Strether tells her the reason of his travel to England: his fiancee, Mrs. Newsome asked him to go there and fetch her son named Chad. There are bad rumors in his hometown about this young man: it is said that he fell in love with a very inappropriate woman for him and, when Mrs. Newsome ordered him to return home immediately, he refused and disappeared. Mrs. Newsome laments that Chad is needed in Woollett to help her run the family business. Lambert sees himself as a honorary ambassador on a noble mission and believes that returning Chad home will lead to the marriage. Miss Gostrey carefully says that Lambert can’t be sure that Chad is wrong until he hears the story from his side and knows him personally.
Soon, Waymarsh arrives to the hotel and he, Strether and Miss Gostrey, who decided to join them in their quest travel to Paris where, according to their information, Chad lives with his beloved. Miss Gostrey is lively and cheerful and she turns the boring and ordinary travel of two older men into a tremendous journey with lots of places to see, visit and have fun. Finally they arrive to Paris and find Chad’s residence, but learn that he is away to travel and no one knows when he returns. While they wait for him, Miss Gostrey continues to entertain them, showing them interesting things and places in Paris and around the city. Strether, who at first took his mission just as noble duty, starts to actually enjoy staying in Europe with friends.
Once, he walks past Chad’s house and sees an unfamiliar man standing on the balcony. It is definitely not Chad, but Strether talks to him and learns that his name is “Little Bilham” and he is Chad’s friend who looks after his house while Chad is travelling. Miss Gostrey feels that there is something wrong with this man and suggests that “Little Bilham” hides Chad and acts accordingly to his and Chad’s plan. But Strether thinks otherwise: he finds Little Bilham a decent young man and befriends him, even inviting him to the opera. But Bilham doesn’t come, another young man with the ticket enters his box in the opers. Lambert realises that this stranger is Chad Newsome. Chad has returned from his travel and he looks and acts different to his mother’s description. Now Chad seems to be much more mature, he is a completely changed man. Lambert greets him and invites to watch the opera which both of them enjoy greatly.
After the opera ends, Lambert tells his guest about the reasons that brought him to Paris: about Chad’s mother struggling without him and worrying about his fate and also about Lambert’s own matrimonial plans. However, while Strether speaks, his belief that he is doing the right thing dwindles. He feels that Chad is a grown-up man, not a delinquent child that should be returned home, but a person, who is able to take responsibility for his actions and live his own life as he feels is appropriate. Chad is restrained and confident, listening to Lambert patiently and respectfully, impressing the man to no end. Lambert can’t help but wonder, what changed Chad to such an extent or what made his mother make such a huge mistake of considering him just a misbehaving boy. Chad invites Lambert to be his guest and to meet his close friends who are going to arrive soon: a family consisting of a mother and a daughter. Strether assumes that one of these women is Chad’s “inappropriate” lover and wonders which one and if that woman is the cause of his transformation into responsible adult. While they are waiting for the women to arrive, Lambert continues talking to Little Bilham and learns from his words that Chad’s relationship with his beloved woman are purely platonic and innocent. Finally the women arrive and Lambert is introduced to them on a high society gala, but their meeting is too brief for him to make any conclusions. He learns that the name of the mother is Madame de Vionnet and the name of the daughter is Jeanne. Their meeting lasts mere minutes and then Lambert is left alone with Little Bilham. Touched by all the joy he experienced during the trip with Miss Gostrey and here, in Paris, Lambert starts a sentimental talk, advising the young man to live to its fullest until it is too late. This advice Lambert gives also to himself: he understands that in Paris he relives the years of his youth, feeling more alive than ever in his life.
The time passes and Strether with his friends spend more and more time with Chad, Little Bilham and Madame de Vionnet and Jeanne. While knowing them all better, Lambert makes a conclusion that Madame de Vionnet is a great mentor for Chad who has a very strong and good influence on him. Suddenly it appears that Miss Gostrey and Madame de Vionnet are old school friends who moved away and haven’t seen each other for many years. They are very glad to reunite again and have a lot to talk about. Soon, Lambert also knows that Jeanne is engaged with another man and Chad actively assists in arranging their marriage. It means that Madame de Vionnet, not Jeanne, is his love (or not love at all, but dearest friend and mentor) that his mother considers scandalous. She is the reason that keeps Chad from returning to America and family business. Strether himself is charmed with the personality of Madame de Vionnet and one day he suddenly realises that he has just promised her to help Chad stay in Paris, near her. To make everything worse, right after this promise Lambert receives a letter from Mrs. Newsome. She wants him to act faster and is very irritated with the delay of her “ambassador”. Mrs. Newsome demands that Lambert either brings Chad home or return to America himself immediately by the next ship. Understanding that his future marriage is endangered, Lambert still firmly refuses to return immediately in his letter, deciding to stay in Europe and enjoy life for some more time.
But unexpectedly Chad declares that he wants to return back to America and start to work for his family business. Bound by his promise, Strether asks him to stay, rationalising his being in Europe. Meanwhile Mrs. Newsome, who is completely disappointed in her first “ambassador”, sends the next party of them: her daughter and Chad’s sister named Sarah Pocock, her husband Jim and Jim’s sister Mamie who Mrs. Newsome considers the proper match to Chad and sends her to Paris to marry him and bring him home. Strether realises that his fake reports about Chad are about to be busted by the new set of witnesses and panics.
At first he hopes that Sarah will be enamored with European life as much as he was but soon they have a serious talk and Lambert understands that Sarah is a copy of Mrs. Newsome with all her conservative views and negative approach to the situation. She even uses the same words as her mother did, while demanding Strether’s obedience. Sarah wants Lambert to convince Chad to return home as soon as their common trip to Switzerland is over. What should be a leisure trip for Sarah, Jim, Mamie and Waymarsh becomes a super-secret timed mission for Strether now. Leaving, Sarah blatantly lets him know that she and Waymarsh are very, very disappointed and upset with Strether’s reckless behaviour.
After they leave, Strether himself goes to the French rural areas just to vent his stress. Suddenly, while walking near the lake, he spots Chad and Madame de Vionnet in a boat in the middle of the lake. Watching them, Lambert realises that they lied about their platonic and innocent relationship: they are clearly intimate. Still, despite they tricked him, Lambert has to admit that love to Madame de Vionnet changed Chad a lot and for good, improving him as a person.
Lambert returns back home and waits for Chad and his lover to return also. When they come back to Paris he visits Madame de Vionnet and confronts her, telling that he knows everything about her affair. The woman seems confused and defeated. She admits that Chad has to end this relationship and return back home to the United States. After a really weird conversation Madame de Vionnet says that she desired Lambert himself all along and he replies that she had him, because he also became attached to her. Despite this turn, Lambert offers her to continue keeping Chad in Paris, near her.
However, while Chad and the others are not around (Chad is also away for the whole week), Strether becomes more and more confused. His determination to cling to Parisian life by keeping Chad here and hiding from his fiancee starts to wear off. When Chad returns to Paris also, Lambert has a final talk to him, demanding from the young man to return home. To his surprise, Chad himself thinks and talks only about his life in America, improving family business and helping his mother in Woollett. He is eager to return home right now. Strether is shocked: he believes wholeheartedly that Chad will be much happier in Europe with such a good woman as Madame de Vionnet, than in America with arrogant and shallow Mamie. But still he can’t convince Chad to stay for more.
Finally leaving both of the couple to deal with their own decisions, Strether returns to Miss Gostrey. He realises that for all the time he was in Paris dealing with love triangles and intrigues, his feelings to her grew stronger and stronger. After his awkward confession, she blatantly makes a marriage proposal to Strether, but he is still too confused. He knows that his life in Europe will be bright and full of joy, but he isn’t ready to accept it. Refusing the proposal of Miss Gostrey, Strether decides to return to Woollett as Chad will do and marry Mrs. Newsome as he planned initially.