By Susan Isaacs

HarperCollins. 412 pp. $21.95

WHEN Susan Isaacs's latest novel, Magic Hour, opens, our hero, Detective Steve Brady, is with his fiance'e, Lynne, in his Bridgehampton, Long Island, backyard. It's three months away from his wedding day and his intended is gorgeous, young, sincere. She'll make a wonderful wife. Trouble is, Brady finds that he has to keep reminding himself of this.

Lynne just isn't any fun. Brady is looking for a verbal sparring partner, someone he can ping with, and Lynne just isn't it. So when headquarters calls to say a murder has occurred, Brady feels "a certain gratitude toward Sy Spencer," the corpse.

Spencer. He represents everything Brady hates. He's a summer person, trendy, rich and, as if that isn't enough, a movie producer to boot. Or was before two bullets dropped him "in his plain white terry cloth bathrobe (which he was too well-bred to have monogrammed), standing on the tile deck of the pool of his beachfront estate, Sandy Court, sipping a glass of iced black-currant tea, talking softly into his portable phone."

Brady is nothing like Spencer. He's a resolute wisecracker, unpretentious, a Bridgehampton year-rounder who lives "in a former migrant worker's shack . . . renovated by an hysterically ambitious, pathetically untalented pony-tailed Brooklyn Heights architect who comprehended, too late, that the place would never be considered A Find."

But despite Brady's personal class war, he is, he tells us, "itching to figure out what kind of guy/gal (I'm an equal opportunity detective) the perpetrator was."

Hardboiled, Brady isn't. The shell is there, but he's soft-boiled, squishy inside. He's a Vietvet, an ex-druggie, a barely recovered alcoholic. And he's the way he is about the influx of Manhattanites because he's spent his "whole life being local color in what people called the Fashionable Hamptons." And this while his brother, Easton, "a pain-in-the-ass prig who ironed his t-shirts,") and his mom, a salesclerk at Saks, would go on and on about his mother's New York friends, who "of course, were not her friends but her customers, summer women who came to the grand old houses, 'cottages' in Southampton -- like the one Sy bought -- for the summer."

Now Easton, who had become Sy's go-fer or, as Easton puts it, "Assistant Producer . . . and swatch-carrier," is one of the suspects. But there are plenty of others, too, ex-wives and movie folk begging to be satirized. Isaacs, using Brady as the mouthpiece, goes after them with acid-etched accuracy. For instance, "the room was so shadowy it was hard to make out her feature -- except for her teeth. They were double normal human size; it looked as if she'd had a transplant from a thoroughbred mare. Felice was so aggressively unattractive that, considering her surroundings, you knew it was her, and not Mr. Spencer or Mr. Vanderventer, who owned the sixteen-foot-high ceilings and everything under them." At the other extreme, there's Sy's live-in movie star, Lindsay "if your taste ran to devastating blondes she wasn't bad" Keefe.

Enter Bonnie and her dog, Moose. Bonnie is gangly and athletic, lonely and over-the-hill, a failed screenwriter. So Brady, of course, falls in love. This complicates his relationship with Lynne and with the homicide squad as well.

In fact, when Bonnie is deemed the murderess, Brady freaks, shanghaiing her and Moose into his Jaguar and roaring off, "Bonnie in the passenger seat, me in the driver's seat, and fatso Moose stretched over both our laps, barking with pleasure at this wonderful game." Brady explains his aiding and abetting to us: "This is life we're talking about here, not goddam Agatha Christie where Lord Smedley-Bedley's black-sheep cousin gets murdered after crumpets with the vicar on a rainy afternoon."

We root for him and Bonnie even though we aren't exactly sure she hasn't committed the crime. The match, we realize, is made in heaven. Bonnie pings and zings. "I know what (Sy) went for," she tells Brady early on. "A Lindsay, someone breathtaking. Or someone wispy and intellectual and twenty-two from Yale. Or a jet-setty type with a French accent who could quote Racine -- with hair under her arms and a chateau." Even better, Bonnie talks back.

The conclusion? It's clever, unexpected, drum-tight.

For Susan Isaacs, Magic Hour is a kind of harking back to her first novel, the book that made her famous, Compromising Positions. Here, as in that book, the plot is streamlined and the time-frame is short and the voice we hear is witty, and coming-right-at-us real. Because of that voice, we invest something in the characters and their fate. In other words, though she has us laughing very nearly all the while, Susan Isaacs never writes a mere mystery, a murder-tra-la, an exercise in detection, but something more.

Carolyn Banks is the author of four suspense novels.