Lewis Lambert Strether, the protagonist of the novel, is a cultured man in his fifties from the fictional town of Woollett, Massachusetts, who is dispatched to Paris to find Chad, the wayward son of his fiancée Mrs Newsome. The book is entirely told from Strether's point of view and chronicles his change from an American to a European view of things.
Strether, a middle-aged American of insignificant means, is sent to Paris by Mrs. Newsome, his wealthy fiancée. The mission he has been given is to talk her son, Chad, into returning to the family business in Woollett, Massachusetts. The Newsome family believes that Chad might be overstaying his European tour because of an inappropriate romantic liaison, perhaps with a vulgar adventuress. The reader is given to understand, in indirect ways, that if Strether fails, his engagement to Mrs. Newsome is at risk.
Once Strether locates Chad, he is surprised to discover that Chad has improved from when he last knew him in America. Chad exhibits restrained urbanity, elegance and manners. This is not what Strether expected of someone in the grip of an inappropriate romantic entanglement. Strether wonders what has caused the transformation he sees in Chad. When Chad offers to introduce him to some of his close friends -- Madame de Vionnet and her grown daughter -- Strether eagerly accepts. When the introduction occurs, Strether finds the mother and the daughter to be refined, virtuous and thoroughly admirable. He wonders if the lovely daughter is what has brought about the improvements in Chad. He learns that Madame de Vionnet is married but has been separated from her husband for years.
Strether himself is introduced to Paris in a way that starts to open his own mind and heart to a larger vision of the world's possibilities. He feels alive and renewed. His own interest in returning to America wanes. It is also clear that he is not exerting himself to talk Chad into returning. He develops some feelings for Madame de Vionnet. To his surprise, Chad assists in arranging a very advantageous marriage for Madame de Vionnet's daughter. This leaves Strether to wonder what might be going on between Chad and the mother.
Meanwhile, Mrs. Newsome grows tired of Strether's failure to act. She sends over a set of new ambassadors to accomplish the mission -- including her daughter and son-in-law. The group clearly doesn't see Paris, life's possibilities, Chad or Madame de Vionnet in the same way that Strether does. Sarah (Newsome) Pocock demands that Strether get himself in line and insist to Chad that Chad return. While the Pocock party is away on a short trip, Strether ventures out of Paris for a day of random wandering and accidentally happens upon Chad and Madame de Vionnet in a setting where it is evident that they are romantically and sexually entangled. He feels deceived but still acknowledges the improvements in Chad's character.
Chad makes the decision to return to Woollett, which will mean the sundering of his relationship with Madame de Vionnet. Strether has very mixed feelings about this. He, too, will return to Woollett, even if this means a less-alive existence. It is unclear whether Mrs. Newsome will receive him back into her graces.