'The Mortal Instruments: City of Bones' (PG-13)

Start with a heaping helping of Buffy the Vampire Slayer. Throw in some Percy Jackson, a dash of Twilight, a spoonful of The Vampire Diaries and a sprinkling of Harry Potter, and you end up with The Mortal Instruments: City of Bones, the latest in a seemingly endless series of coming-of-age stories set against a fantasy/horror backdrop. In the 1980s and ’90s, teenagers usually served only one purpose in scary movies: To get slaughtered. Today, though, they increasingly make their way into young adulthood by vanquishing supernatural evil. I suppose that’s progress.

This adaptation of Cassandra Clare’s first book in her popular series of young adult novels is wildly ambitious in scope and looks far better than its relatively modest budget of $60 million. The story focuses on Clary (Lily Collins), a teenager living in Brooklyn with her mother who discovers she’s the latest descendant in a line of Shadowhunters, half-human, half-angel creatures ordained to protect humanity from demons, vampires, slimy tentacled things and practically every other monster you can think of (except for zombies; in one of the film’s few traces of wit, Clary learns there is no such thing).

The creatures live among us, invisible to ordinary mortals, and are kept in check by the Shadowhunters, attractive tattooed teens who wear lots of black leather: the angel-faced Jace (Jamie Campbell Bower), who is supposed to be a fearsome warrior but is so pale and skinny you keep wanting to give him something to eat; the surly Alec (Kevin Zegers), who resents Clary’s presence for reasons that are totally not what you think; and the intrepid Isabelle (Jemima West), who is defined primarily by her mean magical whip.

Instead of frontloading City of Bones with one of those expositional voice-overs that explains the movie’s universe to the audience, director Harold Zwart (The Karate Kid remake) and screenwriter Jessica Postigo allow us to gradually discover it through Clary’s eyes. There are neat little touches, such as the revelation that Johann Bach was a Shadowhunter who composed a piece of music that compels any demon that hears it to shed its human disguise. There are some genuinely creepy set pieces, such as a visit to a run-down hotel inhabited entirely by vampires and an encounter with a kindly witch (The Shield’s CCH Pounder) that takes a sudden and scary turn.

City of Bones contains the requisite love triangle, this one involving Jace, Clary and her sweet-natured pal Simon (Robert Sheehan), who doesn’t even try to hide his longstanding crush on her (she thinks of him as a brother). Although it is aimed squarely at teens, the movie has a refreshingly laidback attitude toward sexuality: When the heroes visit a powerful warlock named Magnus (Godfrey Gao), he greets them in his underwear and immediately starts macking on Alec. There are moments of surprisingly effective horror (including a snarling Rottweiler that turns out to be something far more menacing). Lena Headey, as Clary’s mother, gets to throw down against two baddies in a scene that makes her character Cersei Lannister in Game of Thrones seem like a frail dandelion.

But Clare’s almost 500-page book was dense in history and mythology. Even well into its second hour, City of Bones is still having to explain who the evil Valentine (Jonathan Rhys-Meyers) is or why Hodge (Jared Harris), who is to the Shadowhunters what Rupert Giles was to Buffy, is unable to step outdoors. And I haven’t even mentioned the werewolves. Or the portal into another dimension. Or a whole bunch of other stuff.

Near the big finale, as the movie furiously cuts back and forth between a swordfight, flamethrowers, razor-sharp boomerangs, dungeons, demons made of glowing red ash and swarms of evil birds, I completely lost track of what was going on in this nutty, overstuffed picture. But I can’t say I was bored, either. Only the ending, which leaves about five different plotlines dangling a la the carbonite Han Solo in The Empire Strikes Back, was truly irritating. Plans for the second film are already underway, with a targeted release date of 2014 (with Sigourney Weaver joining the cast). But unlike the Harry Potter series, which managed to cross over far beyond the books’ readers, The Mortal Instruments franchise is strictly for the initiated. They will no doubt love it. Everyone else will be flummoxed.

Cast: Lily Collins, Jamie Campbell Bower, Kevin Zegers, Jemima West, Robert Sheehan, Godfrey Gao, Jonathan Rhys Meyers, Lena Headey, Jared Harris, CCH Pounder, Aidan Turner.

Director: Harold Zwart.

Screenwriter: Jessica Postigo. Based on the novel by Cassandra Clare.

Producers: Don Carmody, Robert Kulzer.

A Screen Gems release. Running time: 130 minutes. Vulgar language, violence, gore, frightening demonic imagery, adult themes. Playing at area theaters.