"The Twilight Saga" mercifully reached its end last year, but we still haven't reached the expiration date for shameless imitators looking to pick up where the blood-(and-soul-)sucking franchise left off. Enter "The Mortal Instruments: City of Bones," a peppy mix of earnest romance and supernatural shenanigans that attempts to differentiate itself with a more elaborately conceived and populated fantasy world (shades of "Harry Potter") and a tougher, more self-reliant heroine (shades of "The Hunger Games").

Borrowing from superior franchises gives "Mortal Instruments" a slight edge over recent failed "Twilight" wannabes like "Beautiful Creatures," "The Host," "I Am Number Four" and "Red Riding Hood," but it still comes at a price. All the ho-hum familiarity robs the movie of the one element essential for a successful franchise (even "Twilight"): a sense of its own identity.

The Bella Swan stand-in here is named Clary Fray (Lily Collins), a seemingly normal girl who is actually part of a long line of "Shadowhunters" -- part angel and part human warriors charged with protecting humanity from the demons constantly looking to raise hell on Earth. Clary enters the Shadow World with the help of her own Edward Cullen, Jace (Jamie Campbell Bower), an androgynous would-be heartthrob aptly described at one point as a "dyed blonde wannabe Goth weirdo." That shade is thrown by Clary's very own Jacob -- here he's called Simon (Robert Sheehan) and conceived as a nerdy lapdog who harbors an obvious but unspoken crush on Clary.

Their lopsided love triangle unfolds against a fully loaded landscape of vampires, werewolves, warlocks, angels and demons that less charitable viewers will likely dub "convoluted," but at least keeps the film from slipping into the languid tedium that defined so much of "Twilight." Author Cassandra Clare, who created the series, is clearly invested in worldbuilding -- she's published five "Mortal Instruments" installments and three spinoff novels since 2007. (She also, unsurprisingly, dabbled in both "Harry Potter" and "Lord of the Rings" fan fiction.)

There's simply so much to explore, or at least introduce, that even though director Harald Zwart ("The Karate Kid" remake) and screenwriter Jessica Postigo Paquette ("Tarzan 3D") fail to bring much imagination to the table, they keep the action flowing at a steady pace. Every few minutes delivers another big battle between some combination of supernatural entities, and despite generally sketchy budget-conscious CGI, they're varied enough to prevent the conflicts from becoming one note. No doubt the intention is to appeal to teenage boys as much as teenage girls. In that way, "Mortal Instruments" sort of resembles a YA "Resident Evil" -- the legitimate franchise also comes from distributor Screen Gems and found its niche by delivering a kick-ass heroine and straightforward action on relatively low budgets.

The overriding lack of originality is redeemed somewhat by a healthy dose of sarcasm and a few novel touches -- Jace's best friend Alec (Kevin Zegers) has an unexpected crush that Clary immediately sees through; New York City's most powerful warlock is an Asian male model (Godfrey Gao) who parades around in boxer briefs and flirts with both men and woman; Clary's Caribbean neighbor (CCH Pounder) prominently figures in a memorable action sequence. But for most of its two-hour-plus running time "Mortal Instruments" just chugs along as middling entertainment, never completely boring, and never particularly interesting either.

It's not until the last act that the movie totally falls apart, when the chaotic climax is confined inside a grand cathedral called the Institute and Jonathan Rhys Meyers hams it up as uber-villain Valentine (a deranged shadowhunter who drinks demon blood). He also tries to destroy Clary and Jace's relationship with a deep dark secret that only exposes the risks of staying too faithful to your source material (what's intended as a shocking reveal falls flat because the filmmakers don't really have the courage to sell it).

Until then, the greatest sins are bland characterizations -- Sheehan is an appealing standout and Collins displays more confidence than ever, but the writing fails them both and completely abuses talented supporting players Jared Harris (as the Institute's shady leader) and Lena Headey (as Clary's kidnapped mother) -- and mundane dialogue like "I'm a shadowhunter Clary, and I'll protect you with my life!"

Speaking of "mundane," that's the film's designated term for anyone not a part of the Shadow World (kinda like Muggles in "Harry Potter"). It's also a word that will surely be overused in reviews blasting "Mortal Instruments." It's just too bad the movie earns every cheap shot.