ONCE UPON A TIME, there was a beautiful young woman named Margaret who grew up and married a handsome prime minister named Pierre. They lived a wonderful life filled with glamorous people and terrific vacations. Together they quickly had three sons. Very quickly. And six years later, it all became too, too much for Maggie and she ran away from home.

Margaret Trudeau became our most famous runaway mother, living out the fantasy so many of us have when the children get too obnoxous, when the husband's job gets too demanding on our lives, when we feel overwhelmed with the needs of our families and no one seems to give a hoot about us. Instead of staying at home and gritting her teeth and saying this, too, shall pass, instead of taking a drink or popping a pill and feeling sorry for herself, Maggie took off.

She was the runaway mother in all of us, and a symbol of that aspect of the women's movement that made it respectable for women to choose options other than motherhood. She and Pierre Trudeau epitomized the family stresses of the seventies, the couples struggling to find answers to new problems, to balance the needs of two adults within a marriage, of couples unwilling to be unhappy, yet unwilling to be divorced.

Theirs was to be a most civilized separation, he retaining custody of the children while she tried to get custody of herself. "I am a free spirit that must survive in a free world," she told People magazine. "I've had enough of being public property." She wanted freedom. "I am not a weirdo, a wacko or an eccentric for wanting to do good, honest work on a day-to-day basis. I just want to find my individuality." This was 1977 and what Maggie said struck a responsive chord. Some people felt she was merely being selfish, but a lot of other people felt she was liberating herself and cheered her on.

And they were understanding. In an age of uncertainty and shifting values, we seemed reluctant to condemn. Maggie, after all, was simply doing her own thing. But what has she done? She's traipsed around Europe and America with movie and rock stars, she's dabbled in photography and starred in bad movies. She has written a book of memoirs that has nudged its way onto the best-seller list.

And she has become an habitue of Studio 54, the quintessential seventies freak show of cocaine and leather, where New York's publicity seekers go to be seen and photographed and chronicled in the gossip columns.This is where you go when you are public property and want to make sure you stay that way.

If Margaret Trudeau started out as a sympathetic character in search of herself, she ended up as the sweetheart of anything goes. She high-hurdled the fine line between self-fulfillment and self-indulgence. She became a public embarrassment to her husband, for her children and certainly to herself, a flower child dancing her life away in the most hedonistic showcase of Manhattan madness. She became a symbol of a generation so satiated with pleasure that it began devouring itself with the bizarre.

Margaret Trudeau made the columns. Her memoirs were excerpted. Members of her entourage wrote about her. She made the cover of People. Maggie was news because she was the wife of a prime minister. She also was news, said one editor, because you never know what she's going to do next. You never know what she's going to say and invariably you are left with your mouth open, saying you can't believe she said that. She has shock value. You never know who she's going to show up with, what part of her anatomy she's going to praise, what private thrill she's going to discuss in public.

There is something sad about Margaret Trudeau. This Sunday, the Washington Star carried an item about a television interview in which she revealed that Prince Charles was her "buddy" and thanked him for "battling at every frontier for my name and honor . . . What woman could be luckier?"

But there also is something terribly annoying about her. Take, for example, the night Pierre lost the election. The news reports the following day told us that Margaret had boogied the night away at Studio 54, the third night in a row she'd shown up. Pierre, she told reporters, is "going to be the greatest leader of the opposition, not only because he is going to fight for individual freedom but because he's going to fight boringness." Of her husband, she said: "I never left him. I just came out to get my freedom." And she complained that she couldn't get a flight to Canada. The UPI story concludes: "As two unidentified men pulled her closer to the bar, Trudeau turned her head and added, 'Maybe I'll have to walk or dance.'"

It is hard to imagine anything more gauche for Margaret Trudeau to have done than to have shown up at Studio 54 that night, dressed in white disco pants and stiletto heels. It is hard to imagine any move much more thoughtless to have done to her husband.

Pierre Trudeau's defeat has moved Margaret a giant step away from political power which certainly ought to make her antics a lot less news-worthy. For a long time, she was able to get away with her behavior because she was the wife of the Canadian prime minister. Somehow that gave doing her own thing a note of respectability. His title protected her. She got sympathy for behavior that would have been scorned in others.

Margaret Trudeau, after all, had three children in four years. She found herself submerged and suffocating in an existence many women would suffer gladly, but she relinquished her husband and her children in a time when we were bending over backwards to be tolerant of other people's behavior. If Maggie wanted to do her own thing, to develop a profession and give up custody of her children, who were we to condemn? Maggie would be a liberated woman of the seventies. She would commute home regularly to Canada from New York to visit her husband and children and meanwhile, she would accomplish meaningful things on her own.

That, of course, is not what happened at all. Somewhere along the way, liberated became confused with libertine, doing your own thing, with doing nothing worthwhile. And through it all, Margaret Trudeau was shielded and protected by her husband's title.

Now Maggie's on her own. She is merely the estranged wife of a former Canadian prime minister, which doesn't quite have the same ring to it. Soon, she may find out that doing your own thing is different from doing whatever crosses your mind. CAPTION: Picture, Margaret Trudeau at New York discotheque Studio 54, By Felice Quinto for The Washington Post