Lambert Strether, a middle-aged yet not broadly-experienced man agrees to take on a mission for his wealthy fiancee: to go to Paris and rescue her son Chad Newsome from the clutches of a presumably wicked woman. On his journey, Strether stops in England and meets Maria Gostrey, an American woman who has lived in Paris for many years. Her cynical wit and worldly-wise opinions start to rattle Strether's preconceived view of the situation.
In Paris, Strether meets Chad and is impressed by the much greater sophistication he seems to have gained during his years in Europe. Chad takes him to a garden party where Strether meets Marie de Vionnet, a lovely woman of impeccable manners, separated from her reportedly obnoxious husband, and her exquisite daughter Jeanne. Strether is confused as to whether Chad is more attracted to the mother or the daughter.
All these impressions of Parisian culture bring Strether to confide in Little Bilham, a friend of Chad's, that he might have missed the best life has to offer. Strether starts to delight in the loveliness of Paris and actually stops Chad from returning to America. Meanwhile, Mrs. Newsome, Strether's fiancée and Chad's mother, waiting impatiently back in the States, enlists new "ambassadors" to bring back Chad forthwith.
Chad's sister Sarah Pocock, the most important of the new emissaries, harshly dismisses Strether's impression that Chad has improved and condemns Marie as an indecent woman. She demands that Chad immediately return to the family business in America.
Strether takes a small tour in the French countryside to escape these troubles, and accidentally meets Chad and Marie at a rural inn. Strether now realizes the full extent of the pair's romantic involvement. After he returns to Paris he counsels Chad not to leave Marie. But Strether finds that he is no longer comfortable in Europe. He declines what amounts to a marriage proposal from Maria Gostrey and returns to America.
The theme of liberation from a cramped, almost starved emotional life into a far more generous and gracious existence plays throughout The Ambassadors. But it's important to note that James does not naively make Paris into a faultless paradise for stinted Americans. Strether learns about the reverse side of the European coin when he sees how Marie desperately fears losing Chad after all she has done for him. As one critic put it, Strether does not shed his American straitjacket only to be fitted with a more elegant European model.
Instead, Strether learns to evaluate every situation on its merits, without prejudices of any kind. The final lesson of his European experience is not to trust preconceived notions from anybody or anywhere but rather to rely on his own observation and judgment.
Critical controversy has swirled over Strether's refusal of Maria Gostrey, with some seeing it as a perverse rejection of his best chance for happiness. Others have simply said that Strether, while a great friend of Maria's, is not in love with her and the couple couldn't have made a successful marriage. Critics have also speculated about whether Chad will heed Strether's advice to remain with Marie or if he'll return to America for the substantial rewards of the family business. The general verdict is that Chad will follow the money.
In a letter to a friend James said that Strether bears a vague resemblance (though not facial) to his creator. It's true that Strether shows an ability to grow in understanding and good judgment, though some critics have seen him as limited and timid even after his European experiences.
A long-standing literary mystery is the nature of the "little nameless object" made in Woollett. Strether calls it: "a little thing they make—make better, it appears, than other people can, or than other people, at any rate, do"; and he calls the business: "a manufacture that, if it's only properly looked after, may well be on the way to become a monopoly." In an article in Slate, Joshua Glenn argues that it is a toothpick. As Glenn points out, other critics have guessed matches, toilet articles, button hooks, etc.