Joe Carnahan, a 32-year-old screenwriter and director, has a joke he likes to tell about the way opinions are formed in Hollywood. He first heard it from the actor Ray Liotta, and over the past couple of years, in numerous retellings, it has attained such significance to him, such potent explanatory power about how the business works, that he has come to refer to it simply as the Joke. The joke goes like this: Two agents -- or movie producers or studio executives -- are walking down the hall, talking business. One turns to the other and asks, ''So what did you think of that screenplay that came in the other day?'' 

''I don't know,'' the other one says with a shrug. ''I'm the only one who read it.'' 

On Dec. 20, in the heart of what is traditionally Hollywood's prestige-film season, a low-budget police drama called ''Narc,'' written and directed by Carnahan, will open in selected cities and soon thereafter roll out across the country. Set in the most blighted areas of Detroit in the bleak, gray midwinter, ''Narc'' tells the story of two burned-out cops, played by Liotta and Jason Patric, trying to solve the brutal slaying of an undercover narcotics officer. It's a tense and downbeat affair, punctuated by sudden eruptions of violence: a man is stabbed in the neck with a needle, another is shot in the head and a pregnant woman is struck in the belly by a stray bullet, killing her unborn baby, all before the opening credits roll. Nearly every line of dialogue is either shouted or spoken through clenched teeth, and every punch, and there are many, sounds like a baseball bat hitting a brisket. 

Given its grim subject matter, $5 million budget and sub-A-list stars, ''Narc'' is the sort of movie that, artistic merit notwithstanding, would often fail to secure a theatrical release and wind up dumped onto the video store shelf, where its heartbroken director will see a single forlorn copy sitting beside 16 of the Sylvester Stallone car-racing movie. Instead, ''Narc'' is being unfurled with a full promotional press by a major studio, Paramount, on Hollywood's all-important Christmas weekend -- right into the mouth of the hydra,'' Patric says. Harrison Ford has already signed on to star in Carnahan's next film, and a recent issue of Rolling Stone proclaimed Carnahan a hot director. ''It's like, you're hacking through the jungle with a rusty butter knife,'' Carnahan says, ''and then, suddenly, the jungle clears and you see a landing strip with an F-16 on it, waiting for you.'' 

The story of how a little-known filmmaker with a little movie became such a big deal over a period of just a few months has much to do with the magic wand of star power -- in this case, the wand of Tom Cruise, who saw ''Narc'' last spring and, under the auspices of his production company, brokered its deal at Paramount. (Tom Hanks recently provided similarly critical back-channel support as a producer of the comedy ''My Big Fat Greek Wedding.'') It also has much to do with the communicable, instinctively herding nature of buzz in Hollywood, especially when that buzz is harnessed to power. And in that, Carnahan says, ''it all comes back to the joke.'' 

Like any decent Cinderella story, this one begins with its protagonist toiling in obscurity. A burly, 6-foot-2 grocery-store owner's son from rural Michigan, Carnahan had, until recently, limited expectations for his filmmaking career. He made his first movie five years ago, borrowing equipment on weekends from the Sacramento TV station where he worked editing promotional trailers to shoot a quick and dirty crime story about used-car salesmen called ''Blood Guts Bullets and Octane.'' The entire thing cost around $7,300. ''I did it as a lark,'' Carnahan says, ''just to do it.'' In 1998, after the film played at the Sundance Film Festival, Lions Gate Films, an independent film company, bought it for $150,000 and released it in a small handful of theaters. Few people ever saw it, but many who did glimpsed signs of a promising filmmaker: ''What Carnahan made for his money,'' Roger Ebert wrote, ''is a fabulous calling card.'' Still, in terms of industry buzz, ''Blood Guts Bullets and Octane'' was, essentially, a mosquito. 

Next, Carnahan began shopping around the script for ''Narc,'' which originated as a short film he'd made while a student at San Francisco State. He pitched it as a raw cop drama in the tradition of the 70's films of Sidney Lumet and William Friedkin. The lack of interest was overwhelming. ''I heard it from all quarters,'' Carnahan says. '' 'This is an ''N.Y.P.D. Blue'' episode.' 'Nobody cares about the cop genre anymore.' '' 

In time, the script landed in the hands of Liotta, who had recently moved over to Carnahan's agency, Endeavor, in hopes of recharging his slumping career. ''I was looking for a movie that would have some impact,'' Liotta says, ''or at least make some cash.'' The actor set up a meeting with Carnahan. ''Joe said, 'I want to do this as a Cassavetes-meets-French Connection'' kind of thing.' You could tell he'd done his homework. I thought, I want this guy to get a shot.'' Liotta agreed both to star in the film and to produce it. 

For a year, the lights in the movie's path remained red. Few production companies were eager to throw millions of dollars at a violent Ray Liotta cop flick, and those that did express some interest had their own casting ideas. ''I remember hearing a particular name, I think it was a Baldwin brother: 'If you get this guy, you'll get three shelves at Blockbuster,' '' Carnahan says. ''I wanted to put a gun to my head.'' Finally, an independent outfit called Cutting Edge Entertainment agreed to produce ''Narc,'' and in February 2000 Carnahan began a 28-day shoot in Toronto. 

Within two weeks of filming, the movie's financing began drying up, and Carnahan and his stars were forced to defer their salaries. ''Every day I got up and didn't know if we'd be shooting or shutting down,'' Diane Nabatoff, a producer on the film, recalls. One evening in a bar, Liotta bumped into Michael Douglas and the producer Arnold Kopelson, who had heard rumors of the movie's problems. '' 'You've got to walk,' '' Liotta remembers Douglas saying. '' 'Get out of there.' '' Within a week or so, new investors surfaced, handing over checks in exchange for producer credits. In the end, there would be 17 producers listed on ''Narc,'' only a handful of whom Carnahan actually knew. 

Last winter, Lions Gate, which had bought the film's domestic distribution rights for close to $2 million, submitted it to Sundance, where it was accepted into the dramatic competition. In style and subject, ''Narc'' stood out from the festival field and drew considerable notice. ''People were surprised how much they liked it,'' John Cooper, Sundance's director of programming, says. ''You think: police drama, Ray Liotta, Jason Patric -- whatever. But the audience felt like they were watching a director who knew what he was doing.'' Nevertheless, when the awards were handed out, ''Narc'' was passed over. '' 'Reservoir Dogs' didn't win anything its year either,'' Cooper says. The Sundance crowd, Patric adds, tends to favor ''movies about lesbian ex-nuns who hand out pamphlets in the ghetto, shot on digital video.'' 

Following Sundance, Lions Gate began to screen the film privately to members of Hollywood's elite on what is commonly referred to as the Bel-Air circuit. Previewing a film well before its release for stars and dealmakers is standard practice and serves a dual purpose: the screener sows seeds of buzz in the industry's richest fertilizer, and the screenee gets an early look at up-and-coming talent. ''People want to know who's young and hip for their own careers,'' Liotta says, ''and Joe was the new guy in town.'' 

One of the first to see ''Narc'' was Dustin Hoffman, another Endeavor client. ''Dustin flipped out,'' Carnahan says. ''I spent like three and a half hours talking to him.'' Ron Meyer, the head of Universal Studios, screened the movie at his house; Warren Beatty was among the guests, and he also called to voice his enthusiasm. David Geffen asked to see the film. ''Pretty soon,'' Carnahan says, ''I'm sitting in a room with people a scant few months before I wouldn't have been able to get within 10 feet of.'' Over lunch, Carnahan says, William Friedkin told him ''Narc'' was ''the best cop film ever.'' 

In March, after ''Narc'' had been making the rounds for a month and a half, Cruise's producing partner, Paula Wagner, called Lions Gate and requested the film. After seeing it, she phoned Cruise and urged him to watch it. ''I was absolutely blown away,'' Cruise says. ''To see something so fresh -- it was exhilarating, really. I called Steven Spielberg and said, 'You have to see this film.' And I called Paula and said, 'I'd like to meet Joe Carnahan.' '' 

They set up a meeting, and Cruise's enthusiasm for the film was obvious. ''He couldn't have gotten it any more thoroughly,'' Carnahan says. ''He was pointing out particular shots, elucidating things.'' Carnahan asked Cruise if he could lend his support, and Cruise agreed. 

Cruise and Wagner have been closely aligned with Paramount since establishing their production company, Cruise-Wagner Productions, there in 1993. They attached themselves to ''Narc'' as executive producers and asked the studio to distribute it. ''This was a rather unique situation for us,'' Wagner says, ''to be like -- what's the right word? -- like godparents, in a way. But if Tom Cruise's name can help call attention to a film that is so deserving, great.'' 

''We're trying to be the angel over the shoulder, really,'' Cruise says. ''What a studio like Paramount can do is guarantee the theaters, hold them for as long as possible and spend the kind of money that should be spent.'' 

Paramount acquisition executives first scouted ''Narc'' around the time of Sundance, with an eye toward possibly buying its foreign distribution rights. But as Rob Friedman, vice chairman of Paramount, says, ''Things definitely heated up when Tom and Paula were excited about it.'' It also didn't hurt that Friedkin, who'd been so enamored of the film, is married to the chairwoman of Paramount, Sherry Lansing. Within two weeks of Cruise's seeing the film, a deal was in place for Paramount to acquire domestic rights from Lions Gate for about $3 million. (Technically, the deal is a partnership; Lions Gate remains involved on a consulting basis and will get a percentage of the film's grosses. But, as Liotta says, ''It's Paramount's ball and bat and dollar.'') ''It was just like bam, bam, bam,'' Carnahan says. ''Suddenly I heard, 'Paramount's buying the film, and Tom's the reason.' '' 

One person familiar with the deal says that Cruise and Paramount's interests in ''Narc'' go beyond pure love of the film: ''One of the absurdities nobody seems to blink an eye at is that Cruise and Wagner saw the movie after it was completed and attached themselves as executive producers, as if they had something to do with making it. They want to be associated with cool, cutting-edge entertainment, just to brand themselves. And Paramount is in this mostly for their relationship with Tom Cruise.'' 

Cruise insists he's only helping out as a fan. ''I even said to Joe: 'Look, you don't have to give us credit. We don't want credit on this movie.' But they wanted us to do it to try to get some attention for the picture. We literally get nothing out of this outside of the satisfaction of putting the movie in the best position it can possibly be in.'' 

Paramount based its decision to release ''Narc'' in December on the theory that, while a dark, violent, marquee-starless genre film may not offer the usual easy publicity hooks (Ray Liotta is an unlikely cover subject for, say, Vanity Fair), if it garners enough critics associations' awards and makes enough 10-best lists, audiences will be curious what all the fuss is about. ''December is a time when the critical community's antennae are particularly sensitive,'' Friedman says. ''We think 'Narc' is a big Academy Awards picture that'll be heavily considered in many categories. That process starts now: building the reputation of the movie, letting people discover it.'' The ultimate hope, Friedman says, is that ''Narc'' will be ''the classic case of the little engine that could.'' 

Much to Carnahan's amazement, in a few short months a low-profile project that had once been deemed, by consensus opinion, to be cinema non grata has been transformed, by consensus opinion, into a highly anticipated awards film. ''Hollywood is the land of the nodding heads,'' the director says. ''All the tributaries empty into the vast sea of perception. Having the muscle that is Tom Cruise and Paramount behind you changes people's perception, puts the movie in a new light. It's like having the most popular kid in school throw his arm around you and say, 'You're my new best friend.' '' 

Soon after the Paramount deal was sealed, Harrison Ford attended a screening of ''Narc'' in New York. On May 9, Carnahan's birthday, the director received a call from Ford, raving about the film. Less than a week later, Variety reported that Carnahan would direct Ford in a crime thriller, ''A Walk Among the Tombstones,'' for Universal. Ford would later be quoted in the press as calling ''Narc'' ''the best movie I've ever seen.'' Carnahan is set to begin shooting ''A Walk Among the Tombstones'' early next year, after which he plans to direct an adaptation of Mark Bowden's best-selling book ''Killing Pablo.'' ''If Ray had never come along,'' he says, ''I'd be looking for, like, the Mary-Kate and Ashley Olsen erotic-thriller vehicle now.'' 

In Hollywood, fairy tales are often written in disappearing ink; in the end, Carnahan says, it's the work, and the hunger for the next gig, that make a career. ''Look, I'm glad I can get my phone calls returned,'' he says. ''But you know what? It ain't for lack of trying. While buddies of mine were going to keggers Friday night, I was staying home writing scripts that I'd print out and put in my closet. Now those guys I used to know are going, 'Hey, dude, I've got a screenplay.' ''