If you’re a “Rocky Horror Picture Show” virgin, I’ll let you in on a dirty little secret: It’s half an hour of quality movie, and the rest is a good-natured mess. This is true of both the 1975 cult musical that became a midnight-show rite of passage for millions, and Fox’s update, notable for making its “sweet transvestite from transsexual Transylvania” an actual transsexual actress, the great Laverne Cox (“Orange is the New Black.”) In the role of mad scientist Dr. Frank-N-Furter, Cox takes over for Tim Curry and puts her own stamp on the role — no small task, given Curry’s indelibly suggestive snarl. Sporting a glam red wig and elaborate fishnets on those long legs, Cox gets the job done in high vamp mode, even if her singing chops aren’t always quite up to par. The whisper of a story revolves around goody-two-shoes couple Brad (Ryan McCarten) and Janet (Victoria Justice), who end up in a remote castle on a rainy night after their car gets a flat. They quickly find themselves in a throng of singing, “Time Warp”-dancing, kink-loving servants and party guests, whose leader (Cox) is a walking id with designs on playing God. Her lust-fueled creation, the eponymous Rocky (Staz Nair), is zapped to life by lightning (a Mary Shelley gravestone in the opening song is a nice wink), and spends the rest of the film running around in just a pair of sparkly gold shorts — less revealing ones, it must be said, than those on 1975’s “Rocky Horror” movie. Annaleigh Ashford and Adam Lambert rock out.Photo: Steve Wilkie/FOX For all its attempts to be outré, this version feels, well, a little tame. But that’s inevitable in a world where, thankfully, “transsexual” doesn’t register as exotic anymore. (Our star, let’s remember, is an Emmy nominee and a Time magazine cover girl.) Plus, stacking it up against its forebear is beside the point: This is a “Rocky Horror” for a new generation, one that was introduced to the rockabilly pelvic thrusting of “Grease Live!” just last year. Baby steps. Director Kenny Ortega does his best to showcase the musical’s real appeal, which is, of course, audience participation. Every so often he cuts to a leather-clad movie theater crowd, yelling beloved talk-back lines from the live shows. Meanwhile, Curry steps in as the elderly narrator, giving us a bemused gaze over the tops of his gramps glasses. Younger cast members are in top form, too: Reeve Carney does a high-voltage Riff Raff, while Christina Milian masters an evil cackle as his sister Magenta. Adam Lambert steps into Meatloaf’s shoes as Eddie, the ill-fated biker. Annaleigh Ashford is all sass as the Brooklyn-accented Columbia, and Ben Vereen makes a cameo as wheelchair-bound professor Dr. Everett Von Scott. The high production values here make a pretty solid case for networks spending more time doing musicals right, and dropping the not-so-novel “live!” party trick. Besides, if this “Rocky” has done its job, some viewers will seek out the real deal, which continues to play on in theaters around the world.