Jamie Campbell Bower as Jace, right, and Lily Collins as Clary in a scene from "The Mortal Instruments: City of Bones.” (Rafy Photography)

Sony Pictures didn’t wait for the release of “ The Mortal Instruments: City of Bones ” to announce plans for a sequel. Let’s hope the filmmakers learn at least a few lessons from this first adaptation of the popular teen fantasy series of novels.

To be fair, there are elements worth celebrating. The movie is thankfully less self-serious than the mopey “Twilight” films. “The Mortal Instruments” revels in its own camp.

But there is plenty of room for improvement. The action flick is overly long, complicated and, even by teen romance standards, cringe-worthy in its cheesiness.

Based on a series of novels by Cassandra Clare, “The Mortal Instruments” story feels like a mash-up of “Twilight” and “Harry Potter.” The main protagonist is Clary (Lily Collins), a seemingly typical teenager who begins drawing strange symbols in her sleep and seeing violent images that her best friend, Simon (Robert Sheehan), cannot. It turns out Clary is a shadow hunter, or a half-human, half-angel predestined to track and kill demons. Her realization coincides with the disappearance of her mother (Lena Headey), another such vigilante who’s been masquerading as a bohemian artist in Brooklyn.

To find her mom, Clary teams up with Simon and another shadow hunter, the snarky, smoldering Jace (Jamie Campbell Bower). Their quest coincides with the demon-killer network’s larger goal of finding a magical cup that could save or destroy humanity; Clary’s mother was the only person who knew its whereabouts, but the information may also be hiding somewhere in Clary’s cloudy memory. The plot gets more convoluted from there, with vampires, werewolves, witches and grotesque, oozy monsters joining the mix. The special effects are good for the most part, and the chase scenes are thrilling. If only it were clearer what everyone was running from and why.

But one narrative is easy to follow: Clary and Jace are falling in love, while Simon is also enamored of Clary and shadow hunter Alec (Kevin Zegers) has eyes for Jace.

The characters are plucky and quick-witted, which is a welcome change from the brooding Edward Cullen and monotone Bella Swan. Simon has a wry sense of humor, and Jace offers up his hokey lines with a knowing wink. Even Clary admits at one point the puzzling intricacies of the plot when she says, “This is so confusing.” If only she’d acknowledged the corniness when she and Jace share an intimate late-night moment in a greenhouse amid blooming computer-generated flowers and twinkling lights.

When an overhead sprinkler system turns on, drenching the pair during their first kiss, is it a sly nod to romantic shlock or just another example of it? There were loads of laughs during an early screening of the film, but it was difficult to tell whether the movie was vying for them or not.

“The Mortal Instruments: City of Bones”


PG-13. At area theaters. Contains fantasy violence and action and some suggestive content. 130 minutes.