Kate Croy wants to get married to Merton Densher. The only problem is that Merton is a penniless writer and Kate's Aunt Maud won't support Kate financially if she marries Merton. Enter Milly Theale, an extremely rich young lady from New York who develops a crush on Merton when they meet in America. Milly finds out from her doctor that she is ill (maybe even terminally ill) and she decides to take a trip to Europe with her older, overbearing friend Susan Stringham to get some healthy fresh air.
It's not long before Milly wants to go to London to see if Merton is back in town. When they get to London, Milly and Susan go see Aunt Maud, who happens to be an old school friend of Susan's. Milly finds out that Aunt Maud and her lovely niece, Kate Croy, know Merton Densher. When Merton gets back from America, though, Milly sees him and Kate hanging out alone in an art gallery, and suspects that they're romantically involved. Her friend Susan, though, guarantees her that they aren't.
How can Susan guarantee this, you ask? Well here's the deal. Aunt Maud has become aware of Milly's crush on Merton. She has also become aware of Milly's fortune and her terminal illness. Therefore, Aunt Maud tries to push Merton toward Milly so that he can inherit her fortune and stop pursuing her niece Kate. Eventually, Milly and Susan take a trip to Italy, and Maud, Kate, and Merton follow along. Soon, though, Maud departs back to London with Kate, leaving Merton to propose to Milly. Kate has also been working toward this moment, but only because she wants Merton to inherit Milly's money so he can then have enough to marry her.
While all of this is happening, Merton has a huge crisis of conscience. He can't find his way to swindling a poor dying woman who loves him, and he eventually ruins the plan. He comes home to find out that Milly has died in Italy, and—get this—has left him her money anyway. She knew he was swindling her, but her heart was so pure that she forgave him. Merton, though, feels very guilty about the money. In the book's final scene, he tells Kate that if she wants to marry him, she'll have to do it without the money. If she marries him, he'll turn down the money. If she rejects him, he'll give it all to her. Henry James, though, leaves us totally in the dark about what decision Kate eventually makes.
Dontcha just love those ambiguous endings?