The main charactersin "Impostors," George V. Higgins' lightly plotted new novel, are a notorious media mogul, Mark Baldwin, whose satyriasis and power lust are nearly as famous in Massachusetts as Larry Bird's point average, and a woman gumshoe who casually reveals intimate information about her private life to perfect strangers. These people aren't fooling anybody. "Impostors" is a great title for some other novel.

Not that there aren't plenty of secrets to keep covered up. Twenty years ago Baldwin bought his way out of a statutory rape charge. Bristol County DA Billy Taves would like to forget a botched murder investigation involving Mel Shaw, Baldwin's former bureau chief in the town of Waterford. The Waterford town fathers would just as soon not have too many questions asked about the two decades of crooked land deals covered up by Shaw when he was bureau chief.

Joe Logan, a local TV anchor who is about to go on trial for murder, is threatening to stir up this muck. Logan is charged with using three double-O shotgun loads to blow away a drunk driver who received a slap on the wrist after killing Logan's wife and son in a highway accident. Logan, who took the law into his own hands because he believes the system is too corrupt to dispense justice, is threatening to bring Taves down with him. Seeking a powerful ally, Taves convinces Baldwin that Logan may go after him, too.

Baldwin hires legal investigator Connie Gates on a generous retainer to profile Logan for one of his periodicals. He really wants Gates to find out what information the former anchor has and what he plans to do with it. Gates is suspicious of Baldwin but on advice of counsel, Baldwin's gay cousin Richard Pond, she signs on; she needs the 20 grand.

As we've come to expect from Higgins there is more, all of it characteristically seedy -- most of it in a subplot involving illicit heterosexual activity, covert homosexuality and the suicide of one of Baldwin's sexual companions. Unfortunately, little of it is compelling. None of the characters has much at stake, for one thing. Logan, though not looking forward to prison, is at peace with himself. Logan may have information that, if made public, will give Baldwin some trouble with his board of directors and make Taves' reelection campaign more difficult. Gates' sex life is a mess, but no more than most people's. Baldwin's suicidal girlfriend has problems, but they become important only as they inconvenience him.

Higgins is celebrated for his flawless ear for dialogue, and he's lost none of his sense of pitch. Here's the DA on Joe Logan: " 'Now,' Taves said. 'I'm not saying I can't understand, a guy's wife and son get wiped out by a guy that everybody ever met him knew was sooner or later gonna kill some people driving. Of course I can understand. Thing like that'd wreck me, too. But what it did to him, Mark, cripes -- it was like it made him into a different person, all right? He don't, he's decided all those years he was playing by the rules, all right? Living the way he's supposed to, good to his family, keeps his word, never does nobody any harm? He decided now he was being a jerk. Now, now he thinks the whole thing's a joke. That when I prosecute somebody, it's some kind of act I'm putting on. That when the judges sentence somebody, it's all just a big show. See what I mean? And he's going to get even. With everybody. All at once.' "

On the other hand, although Higgins has developed a marvelous literary equivalent of speech, his dialogue is not always securely motivated. He knows how people talk, but he doesn't always know what they are apt to say or when they are apt to say it. Too often in "Impostors," the words Higgins' people speak are directed more at advancing plot than at revealing character, a problem not so evident in previous books.

George V. Higgins' 15th book fails to deliver on its promises. In it, the mighty are set up for a fall but in the end instead of getting their due or weaseling out in some convincing way they are let gently off the hook. Readers who come to "Impostors" seeking either a well-plotted story or what the publisher describes as a "devastating portrait of corruption in contemporary American life" will be let down. Higgins has had his ups and downs before. In "Impostors," he is attempting to stretch his considerable abilities. Look for something special his next time out of the chute.