By Kim Wozencraft

Random House. 260 pp. $18.95

HYPOCRISY ALWAYS gets a bad rap despite its usefulness. The human gift for thinking one thing while doing or saying the opposite is the lubricant that allows millions of us aggressive predators to live in cramped proximity without constant bloody mayhem on every street corner. We invent euphemisms endlessly, calling it socialization, or civilization, or self-restraint. But they all mean the blessed hypocrisy that keeps us from braining the woman who sneaks ahead in the supermarket line or jamming a ballpoint pen through the boss's heart. A totally honest human would be pathologically dangerous. A large population of such creatures would make our current crime rate seem pablum peaceful.

Yet, we're uneasy with this valuable technique. We use its real name only for its nastier incarnations -- the big and small con games that victimize us, the good intentions gone disastrously haywire. Rush, the first novel by former undercover narcotics police officer Kim Wozencraft, explores the functional range of hypocrisy inch by inch, from soothing benefits to crucial necessity, from self-serving pettiness to malignant catastrophes.

Rush has the tang of a war story as much as a cop book. It is couched in the form of a dark memoir and written with the brisk, torchy compulsion of a thriller. The war of Rush is "the war against drugs" and the whole deception magillah is its terrain. It is the tale of a good guy turning bad and the guy in this case is female police offer Kristen Cates, an idealistic rookie recruited for her youth and her ignorance to work as an undercover narcotics agent for a city police force in Texas. To bust dope dealers she must buy drugs from them. To buy, and not get killed in the process, she must convince the dealers she is not a cop. That means being friendly and using the drugs with them. At first she believes she'll be taught to "simulate" drug use during the buys as easily as she pretends friendship. She soon learns differently. "Simulation is a word that comes in handy in court," her partner tells her. "We're out there to buy dope." The narc survivors's rule is to "be anything you have to be to make the case and keep your ass from getting shot."

Kristen Cates falls in love with the detective who trains her, and she gets to liking the drugs. Cocaine becomes, "The Immaculate Conception in powder form. And I was doing it because someone had to. I was making this personal sacrifice. I was rationalizing . . ."

When she balks at lying in court about her drug use, her detective boss explains reality, "Look," he said, "everybody knows what goes down. All the way up to the judge, they know how it works. But nobody wants to hear about it . . . And no, it isn't the whole . . . truth so help you God, but it takes the dealers off the streets."

The layers of deception and carefully covered posteriors twist in escalating complexity. The risk of death, of addiction, of maimed minds, is constant for the narcs but deliberately ignored by those who order the work and its results.

Wozencraft refers to the narcotics agents as being "under," like divers linked to the legitimate world by a thin hose of air while they search the sewers of the city. They are "pulled up" to their surface cop identity at the end of an investigation, only to find that their sympathies are no longer for straight hypocritical society. They have become the kind of junkies and dealers they are paid to catch.

By then, of course, they are expendable outcasts -- hated by their victims, and dangerous to the bureaucrats who used them. At this point Cates and her lover-partner hit the ugliest wall of all and the racing plot of Rush accelerates for the final curve.

Wozencraft provides police procedure and arcane drug practices in crisp, punchy detail. Her lean, athletic prose is matched by the lucid speed and economy of the narrative. Although no other character in the novel manages to develop beyond two dimensions, the blood-and-brain presence of Officer Kirsten Cates is utterly convincing.

Deliberate hypocrites are fairly rare. Most of us fool ourselves first. Cates is consciously, shrewdly picking her ploys. Whether she's deceiving her parents about what she's up to, manipulating the results of a polygraph test, or playing spaced-out junkie to snare a dealer, Cates's voice has the hard snap of the professional liar who decides, for once, to spill the facts.

Fifty years from now this street hip language of lies, and the specific drugs that are the focus of the novel, may well be unfamiliar to most readers. For now, in this era of denials, cover-ups and cut-outs, Rush smacks of the truth.

Katherine Dunn's latest novel is "Geek Love."