BOSTON — The plot is right out of Hollywood. Make a sexually explicit $40-million movie called ''Showgirls.'' Give it a seldom-used rating of NC-17, successor to the old X label. Announce that rules to ban teens from theaters showing the film will be strictly enforced. Then release the film in the thicket of a presidential campaign where Hollywood's violent and sex-filled movies are denounced for ''debasing the nation.'' The resulting hype, attention, and controversy could lead ''Showgirls'' to an ending where the producers ride into the marketing sunset with big box-office receipts. And if such a commercial success should spur a string of new NC-17 films, it could gain a smudged footnote in cinema history. But the release of ''Showgirls'' by MGM/United Artists in some 800 theaters across the United States (taking place today), faces some formidable opposition. Many organizations, including the Christian Coalition and some church and child-advocacy groups, have called for government regulation if Hollywood won't police itself. ''Showgirls'' is likely to trigger even louder debate, though it is supposedly off limits to children. Some critics are asking: What's an NC-17 rated film doing in suburban malls? In the past, studios have been reluctant to back films that might receive the NC-17 rating, but not for explicit-content reasons. The problems accompanying the rating from Hollywood's perspective were twofold: limited theater availability and the refusal of newspapers and TV networks to carry explicit ads - both of which have prevented such films from being commercially viable in the past. But now many theater complexes, usually located in malls where leases often prevent them from showing NC-17 films, are being built as stand-alone buildings by large chains. Adult films are more likely to be welcome there as competition for the movie dollar increases. ''I find these [explicit] films to be offensive and degrading to our culture,'' says Sen. Joseph Lieberman (D) of Connecticut, ''and in some ways endangering our kids. They play to our lesser natures.'' Senator Lieberman is a congressional sponsor of a bill calling for a ''V-chip'' inserted in television sets to bar children from watching violence. ''In my opinion, Showgirls is not pornographic,'' says Travis Reid, executive vice president of Sony Theaters in New York. ''It is a quality, well-produced film. I'm a lot more comfortable that it has an NC-17 rating rather than an R. It's better to clearly label a film with sexual and violent content because the R has a such a wide basis for application.'' MGM/UA's eight-minute promotional video for the film explains the content of the film: ''This sneak preview contains six minutes of the most erotic footage you'll ever see. Leave your inhibitions at the door.'' Out of 1,000 Sony Theaters in the New York area, 50 will offer ''Showgirls,'' about normal for a new film, Mr. Reid says. A new trend might help bolster profits for ''Showgirls.'' According to the Motion Picture Association of America (MPAA), moviegoers between the ages of 40 and 50 are going to movies at almost double the rate compared with the 1980s. And even though PG-rated films are more likely to be megahits, R movies greatly outnumber PG. Two large theater chains, Carmike Cinema in Georgia and Cinemark in Texas, have decided not to exhibit the film. And already the Daily Oklahoman in Oklahoma City and the Fort Worth Star Telegram in Fort Worth, Texas, have said no to ''Showgirls'' ads. But the ads are appearing in many newspapers across the nation. Video rentals and sales often lift a film into profitability. Blockbuster Video, the nation's largest video-store chain, refuses to carry NC-17 films; they do not intend to make an exception for ''Showgirls.'' Reflecting a widely held societal view about explicit sexuality in films, Mike Caruso, a spokesman for Blockbuster, says, ''We don't feel that families coming in our stores should be subjected to what most of society considers to be unacceptable material for public consumption.'' Other video chains don't have such a policy, however. And to counter limited advertising potential, MGM is offering its video preview, on which it spent $250,000, free to anyone over 18 in video chains such as Tower Video and Video Connection. The trailer includes bare breasts, sexually graphic scenes, and violence as a young woman becomes a nightclub dancer in a Las Vegas show. ''Showgirls'' director Paul Verhoeven and writer Joe Eszterhas persuaded MGM to bankroll their film as an NC-17 movie to directly challenge the prevailing view of the ratings. ''I think most parents in America will feel very comfortable that ''Showgirls'' will be barred from their children,'' says Jack Valenti, president of the MPAA, ''and that's what the NC-17 rating is all about.'' Mr. Valenti also says, ''If you make a movie that a lot of people want to see, no rating will hurt you. But if you make a movie that few people want to see, no rating will help you.'' What producers and directors dislike is the taint of hypocrisy they say exists when submitting films to the MPAA for rating. Mr. Verhoeven has said his previous film, ''Basic Instinct,'' started out as an NC-17 film before the MPAA asked for nine cuts to bring it to an R rating. ''Directors and producers don't like the rating system,'' Mr. Valenti says. ''But I don't really care what they think about the rating system as long as the parents of America find it useful.'' Opinion research by MPAA indicates that up to 77 percent of parents with children under 13 believe the ratings are useful. But despite the rating system, access to violent and sexual films can be easy for children and teens through video rentals, cable TV in homes, and lax enforcement of the ratings at theaters. ''We know that kids see these movies whether parents are around or not,'' says Jane Brown, a professor at the School of Journalism and Mass Communication at the University of North Carolina at Chapel Hill. ''Media has become a sex educator in this country,'' she says. ''My concern is that such films present a narrow view of sexuality; women seen as sexual objects in a violent context, not as whole, loving human beings. This is potentially harmful for kids who haven't had their own experiences yet.'' ''As we all know,'' says Bill Kartozian, president of the National Association of Theater Owners, ''a 15- or 16-year-old who is determined to see ''Showgirls'' is tough to keep out. But we'll do our best.'' The studio is paying for extra personnel at theaters to keep out underage teens. ''As a parent, I don't know too many movie houses that are all that good at enforcing the rules,'' says Douglas Besharov, a resident scholar at the American Enterprise Institute in Washington and coordinator of a Sept. 29 conference on ''Sex and Hollywood: What Should Be the Government's Role?'' ''If Showgirls makes a lot of money,'' says Mr. Besharov, ''then the fear of losing box office will be removed, and the dam is burst. NC-17 will become a marketable niche. '' In 1990, Universal released the first NC-17 film, ''Henry and June,'' a racy look at Anais Nin's affairs with writer Henry Miller and his wife. The box-office tally was around $11 million, hardly a beacon call to producers to follow suit. ''These issues around explicit sexuality go back a long way, to the production codes of the 1920s and '30s,'' says George Bluestone, professor of film at Boston University. ''It is almost impossible, from local city councils to the US Supreme Court, to define what is obscene.'' For Mr. Bluestone, and others, the dehumanizing effect of exploitive sexually explicit films is reason to question their value. ''What porno films have in common,'' he says, ''is that the partners are interchangeable, they are not lovers but mechanical mates with no feeling of closeness, or intimacy, a little like painting by the numbers.''