Another week, another vampire. This time it’s the first picture in a proposed franchise based on the “Cirque du Freak” book series by Darren Shan, about a teen who signs on as a member of a traveling circus of human oddities after becoming the half-vampire aide to a full member of the species. He also gets involved in a potential war between peace-loving vampires, who only feed on humans decorously without killing them or turning them into the undead, and their more old-fashioned, murderous brethren, the vampaneze. But we’re about as likely to see a second movie as we are a follow-up to “Lemony Snicket’s A Series of Unfortunate Events” or “The Golden Compass,” to mention only two similar efforts that ran aground on poor audience response.

Mention of “Compass” is especially relevant since it was directed by Chris Weitz, brother of Paul, who helmed this film. In tandem they made some pretty likable films like “About a Boy,” and on his own Paul did a fine job with the underappreciated “In Good Company.” But each has now stumbled venturing alone into the fantasy genre.

“The Vampire’s Assistant” is one of those pictures that try to meld vampire myth with comedy. The one that worked best—at least from a financial standpoint—has been “Buffy the Vampire Slayer,” though to be honest the original movie was pretty bad, and Joss Whedon’s creation flourished only in its television incarnation. But there have been plenty of others—“Love at First Bite,” “Once Bitten” (with Jim Carrey, of all people), “Vampire in Brooklyn” (with Eddie Murphy), “Vamp” (with Grace Jones) and, of course, the two “Fright Night” pictures with Roddy McDowell. The last are the models that “Assistant” most closely resembles, but the first “Night” was much better than this.

The young hero of the title is Darren Shan (Chris Massoglia), a nice, docile high school kid whose best friend is troublemaker Steve (Josh Hutcherson). The two steal off one night to the freak show that comes to town whose performers include Larsten Crespsley (John C. O’Reilly), whom Steve, a fan of the breed, recognizes as a vampire (and will unsuccessfully ask to make him one). Darren’s tastes run more to the big, multi-colored spider Crepsley uses in his act, and he actually runs off with it. In a melee that follows, Steve’s bitten by the arachnid and near death; in order to save his life Darren agrees to let Crepsley turn him into his half-vampire assistant after they fake his death and go off to the circus camp.

But the chance of a tranquil few hundred years is threatened by news that the long-simmering rift between vampire factions, as Crepsley’s friend Gavner Purl (Willem Dafoe) informs him, seems about to erupt anew due to the machinations of the ferocious vampeneze leader Murlaugh (Ray Stevenson) and a Machiavellian human called Mr. Tiny (Michael Cerveris), who possesses occult powers and aims to bring on Armageddon. To accelerate the process he has Murlaugh turn Steve into a vampire to confront his erstwhile pal Darren.

There are many more characters, a whole menagerie of freaks in the circus. They include its owner Mr. Tall (Ken Watanabe); a snake boy and would-be musician named Evra (Patrick Fugit); Madame Truska (Salma Heyek), a combination bearded lady and fortune-teller who’s Crepsley’s girlfriend; a monkey girl named Rachel (Jessica Carlson) who becomes Darren’s; and even some gnomelike CGI creatures that look like an escaped company of Jawas. There’s also much talk about destiny and events foretold in some unnamed book, as well as intimations about how central Darren and Steve are in the process unfolding. But at the close most everything is left obscure, presumably to be doled out in future chapters, like the Harry Potter saga that’s an obvious model here. But it’s a pretty safe bet that anyone who’s interested in finding out the answers will need to consult the printed page instead of waiting for further movies.

The messiness of the story is matched by the uncertainty of execution. Some of the effects are nifty (the spider is fun), but many are mediocre. And whatever tone Weitz was aiming for, he missed by a mile. At times the movie is dumb sitcom (like at the material concerning Darren’s parents), and at others twee drawing-room comedy (as in the Reilly-Dafoe scenes) or aimless dead air (like everything with Fugit); but when the action moves in, things become really nasty (the final battle is brutal rather than exhilarating). As a whole it seems shapeless and flaccid, as though the writers were trying to cram too much into a single feature (a real possibility, as it covers three books in Shan’s series).

And the casting doesn’t help. There are some bright spots, especially Reilly’s fey turn as Crepsley, and Hayek, who has little more than a cameo but milks it for all its worth. Unfortunately, Massoglia makes a blandly anonymous hero (even William Ragsdale, in the “Fright Night” movies, was more charismatic), and Hutcherson could have brought something besides brute aggression to Steve (just recall what Stephen Geoffreys did with a similar character in “Night”). The same’s true of Stevenson’s Murlaugh, who might have stepped out of the “Underworld” series. The worst comes with Cerveris, though. He’s so amateurish that he’s actually reminiscent of Ed Wood’s hulking Tor Johnson.

It’s difficult to know whom “Cirque du Freak” is meant to appeal to, other than fans of the book series. And even they’re likely to be disappointed by a picture that’s all winded wind-up and no dazzling follow-through. Hogwart’s wizards have absolutely nothing to fear from this bunch of sad second-raters.