Philadelphia is about to celebrate a very different Rocky. Forget the boxing ring and the Art Museum steps, and get ready to do the "Time Warp."

South Street's historic Theatre of Living Arts will host a bash Saturday to commemorate four decades of Richard O'Brien's wacky, racy cult classic The Rocky Horror Picture Show. The film, based on O'Brien's Rocky Horror stage musical, was released in the UK 40 years ago Friday. It premiered the following month in the United States.

Since then, Rocky Horror, about a couple who get stranded in a mansion occupied by a transvestite alien and a bevy of dancing buddies, has become a cult classic despite its initial critical panning. Repeated screenings at New York's Waverly Theater in 1976 prompted fans to start dressing up as the characters and interacting with the screen.

To celebrate, Saturday's pinnacle will be a showing of Rocky Horror with a "shadowcast," in which costumed fans stand before a movie screen to lip sync and imitate the film's characters. Other components of the interactive extravaganza include a "virgin sacrifice" - first-time Rocky Horror-goers are given the spotlight - food-throwing (bags of goodies will be sold at the venue for $2, or bring your own; but no rice, hot dogs, prunes, lighters, or water guns, please), and irreverent call-and-response prompted by the songs' lyrics.

In addition, Saturday's anniversary event includes a themed pregame and costume contest at nearby bar Tattooed Mom. All Revved Up, a Meat Loaf tribute band, will play in the crowd when the TLA doors open. (The singer Meat Loaf appears in Rocky Horror as former delivery boy Eddie.)

Rocky Horror's enduring legacy in Philly is thanks, in large part, to the TLA. Its midnight movie screenings made it a locus of the city's punk rock scene in the late 1970s and early '80s.

"It became almost like a weekly community center," said WXPN DJ Robert Drake, a Philly native who moved to Center City after high school. "Everybody met at the TLA for the movie and on the street dressed up in full costume." He used to play Brad because of his bespectacled appearance.

Ray Murray, who sold TLA Entertainment Group in January to start international horror/art film distributor Artsploitation, also cited the TLA's counterculture credo.

"Back then there wasn't a lot of alternative things around," said Murray, who started out as a projectionist at the TLA when he was 17. "[South Street] was a whole other world."

On Saturday, members of Philadelphia Rocky Horror troupe Transylvanian Nipple Productions will don the customary thigh-highs, fishnets, and other salacious accoutrements to embody O'Brien's quirky roles, from straight-laced, newly engaged Brad Majors and Janet Weiss (played by Barry Bostwick and Susan Sarandon in the film) to scintillating, cross-dressing extraterrestrial Dr. Frank-N-Furter (embodied fully by Tim Curry) and his buff, angel-faced sex-doll creation, the eponymous Rocky Horror (Peter Hinwood).

The production company, which rehearses monthly at the Ritz at the Bourse and performed this month at the Monster Mania horror convention in Cherry Hill, is helmed by longtime Rocky Horror vets Erica and Nick, a thirtysomething married couple who go only by their first names to preserve anonymity.

"We're pseudo-celebrities in a Rocky Horror world, and we keep it that way," said Erica, who often portrays castle maid Magenta. "We say we're in our Muggle [non-magical] form when we're not doing Rocky Horror."

Erica and Nick have been doing shadowcasts together since the 1990s, after meeting in high school.

"People online are like, 'When am I going to see Nick out of makeup?' You're not," said Nick, a real-life computer geek who dresses in drag as Frank, the sweet transvestite from Transsexual, Transylvania.

There's a reason they keep themselves behind the veil of anonymity. Rocky Horror gives them a chance to be someone else for awhile, even if that person wears a laced leather vest and garters.

"It's just the message of the movie: Don't dream it, be it," Erica said. "Some people are like, 'What are you doing up there?' We're becoming those characters for an hour and a half."

Production starts at 9 p.m., Theatre of Living Arts, 334 South St. Tickets: $20. Information: 215-922-1011,