At the end of this book, Strether has found out that Chad and Madame de Vionnet are having a sexual relationship and decides that he has to leave beautiful Paris pretty much immediately and head back to America.
Now we know you're probably thinking, so Chad and Madame are having sex. What's the big deal? And the answer is: well, it's complicated. At least, that's what their FB statuses would say. Basically, Strether feels like his entire life in Paris is based on a lie, because he has tried to ask Chad about this before and Chad has hidden the truth from him. On top of that, Strether is still a pretty traditional guy who thinks it's not-so-moral for a guy to have sex with a married woman. That said, he still wants Chad and Madame to stay together at the end of the book.
But maybe the biggest surprise in this book's ending comes when Strether turns down Maria Gostrey's invitation to stay in Paris and maybe even marry her, deciding instead to return home to boring old Woollett. Strether says he has to go back to America out of moral duty now that his Paris experience has been soiled. Maria says he could just decide to be bad for a change and stay with her, but he answers that she wouldn't love him as much if he weren't so moral.
Think about it. It's a real catch-22 and there's no way around it. Maria loves Strether for his principles; but his principles are also the reason he has to leave her.