Having read the book, I now want to see the movie. If it's faithful to the book, the plot will be a tad depressing, but the costumes will be great. Just going by the DVD cover, though, I have a hunch Martin Scorcese took some liberties, starting with the casting, so perhaps the ending isn't quite as melancholy as in the novel.
Basic plot line is the classic lovers' triangle, a nice young man (Newland Archer) torn between two loves -- the boring girl (May Welland) he's engaged to (although he initially doesn't realize just how boring she is; he's young and naive enough to believe that she actually shares his interests in literature and art), and her exotic bohemian cousin (Ellen Olenska), a woman he knew when they were children, but who has in the intervening years lived primarily in Europe and married a Polish count.
The tragedy of their unhappy almost-love affair hinges on several points: first, Newland's engagement to May is announced the same night he sees Ellen for the first time since childhood; second, Ellen is still married to the count and her numerous relatives are horrified by the idea of divorce; and, third, Newland's a spineless twit and about as self-reflexive as a rock. He's infatuated by Ellen, but is too much of a coward to face that fact -- instead he pushes May into persuading her family to move their wedding date up. At one point May actually asks him directly if he's having doubts, does he want to break off the engagement, and, like an idiot, he says no. May turns out to be a lot smarter about where her fiance/husband's head is at than he is; she's observant enough to recognize that Newland has a bad case of the hots for Ellen.
Newland's second major blunder lies in his unwillingness to say what he's actually thinking to anyone. He thinks some fairly scathing thoughts, including some wonderfully funny sarcastic ones, but never opens his mouth. He's an attorney with the law firm representing Ellen's family, so when she says she wants to divorce her husband, Newland instead goes along with the family's wishes and persuades her not to file the petition, a really dumb move on his part considering that at the time he's still single. It's really hard to feel any sympathy for someone who manages to keep sabotaging himself as consistently as he does. He dithers mentally, he marries May, and then when after a couple years of marriage he finally decides that Ellen is the one true love of his life and and he' s ready set to chuck it all and run off to some exotic foreign locale with her (a prospect she's noticeably unenthusiastic about) May announces she's enceinte.
Ellen goes back to Paris, Newland sticks with May, and everyone lives stupifying boring lives for the rest of their natural days. Well, maybe not Ellen -- one gets the impression that she was going to manage to have some fun no matter where she wound up.
The novel is set in 1870s Gilded Age New York, among the wealthy upper class who lived in 5th Avenue mansions, dressed for dinner, and summered at Newport. Scandals are hinted at but never mentioned directly, and family is everything. It's a milieu Wharton knew well, having grown up in it. By the time she wrote The Age of Innocence she was at the top of her game, a successful writer with half a dozen published novels behind her. I've seen Wharton compared with Henry James, but to me she's less contrived and more readable. I enjoyed this book; of the Pulitzer winners I've read so far it's the first one where I can understand why it won.