There is a very simple theme to "An Unmarried Woman": the pain of being left. But to show this in a pure form - or perhaps to minimize its horror - the movie eliminates the usual social factors involved in the breakup of a longtime marriage.

Erica, captivatingly played by Jill Clayburgh, has had a good marriage for 17 years, but now her husband, while protesting that he loves her, wants to marry someone else. It's refreshing to find eliminated from this common situation the tedious question of fault. Some years ago, there would have had to be an explanation - either that the husband was "sick," or that the wife had "let herself go."

The film continues, instead, with her reaction, complete with nausea, spitefulness, hysteria, recklessness, lachrymose blabbiness, and all the other symptoms a normal, nice, thinking person goes through in such circumstances before pulling herself together.

And yet some nice woman in the same situation is going to throw her defunct wedding ring right smack through the movie screen one of these days.

This is because Erica is young, beautiful, sexy and constantly sought after. She has a glamorous part-time job in a Manhattan art gallery, where she meets interesting men all the time, and she can immediately go to work full-time. Her husband is prepared, even anxious, to meet all her financial obligations. Her closet friends are all strong women who have been through this themselves. Her only child is nearly grown up and supportive. In all, it takes her about an hour and a quarter of depression before she falls in love with a more attractive man than her husband, and one who wants to offer her permanence, which she is now above needing.

There is quite a bit here to irritate any middle-aged woman in the suburbs - with a social circle that invites only couples, several small children who think she probably drove Daddy away, no job skills, an exhusband who complains that he can't afford to support his old family because he has an expensive new one, and a noticeable lack of fascinating men eager to take on the whole menage.

Nevertheless, it's a moving, funny, gripping, touching portrait of a special type of misery. The stages of it are acutely analyzed and sensitively portrayed. The writer-director, Paul Mazursky, is good at presenting a certain urban crowd who have the money and vocabulary to drag themselves through all the current emotions, and he does it with a balance of compassion and satire - although it's a mystery why the heroine never belts one of those endless people who keep saying, "It's all right for you to feel pain."