By Chris Nashawaty
August 31, 2013 at 04:00 AM EDT

Hollywood’s fevered quest for the next Twilight or Hunger Games continues with The Mortal Instruments: City of Bones, which is more than just another watered-down tween franchise wannabe. Based on Cassandra Clare’s hugely popular series of young-adult sci-fi/fantasy/romance novels, the overstuffed but decent film stars Lily Collins (Mirror Mirror) as Clary Fray — a seemingly ordinary auburn-haired New York teenager who develops a paranormal gift that allows her to see cryptic symbols invisible to other mortals (called ”mundanes”) like her platonic best friend Simon (Robert Sheehan). What gives? Quite a lot, actually. There’s so much mythology to cover that you might want to strap in.

It turns out that Clary’s mother (Lena Headey) is a Shadowhunter, an underworld race of half-angel warriors who are the last line of defense between our world and the shape-shifting demons who would happily eat our clueless kind for lunch. When Clary’s mother is kidnapped by a gang of demonic goons searching for a magical chalice, Clary joins up with a group of fashionably hip, androgynous Shadowhunters. One of them is the dreamy Jace (Twilight‘s Jamie Campbell Bower), a tattooed, blond bad boy who looks he stepped out of a dewy Spandau Ballet video. (Hey, it’s possible — he’s been alive for a thousand years.) As the film’s heroine, Collins does a good job as an innocent girl-next-door-turned-demon-slaying badass. She juggles rock-’em sock-’em action numbers (clad in thigh-high leather boots) and the kinds of schmaltzy, Harlequin romance scenes that will get the pulses of teen girls the world over racing. It doesn’t hurt that she’s surrounded by the kind of hammy, scenery-chomping Brits (Jared Harris and Jonathan Rhys Meyers) for whom movies like this seem tailor-made.

For the uninitiated, some head-scratching will be inevitable. The thickets of mystical, mumbo-jumbo plot points are hard to keep straight (are the vampires working with or against the werewolves? And where do the warlocks fit in?). But director Harald Zwart (2010’s The Karate Kid) mostly keeps the derivative, CGI-packed City of Bones fizzy and fast on its feet — at least until the final act, which seems to go on forever in its attempt to set up the sequel. And make no mistake, there will be a sequel. Clary may not wind up having the same pop-culture impact as Bella and Katniss, but like it or not, this won’t be the last time you hear from her. B-