Evil has arrived in the small town of Wheelsy in the form of a meteorite carrying deadly cargo. Using tentacles to infect the town’s richest man (Michael Rooker), this all-powerful alien visitor soon transforms itself into a slug-like beast/man, using thousands of smaller slugs to invade and turn the townsfolk into meat-loving zombies. Facing danger from all sides, it’s up to the town sheriff (Nathan Fillion, “Serenity”), his childhood crush (Elizabeth Banks, “The 40 Year-Old Virgin”), and a teenager (Tania Saulnier) to save the world from certain doom.

Writer/director James Gunn got his start in the business with Troma Entertainment, the purveyors of such Z-grade classics as “The Toxic Avenger.” Writing “Tromeo and Juliet,” Gunn learned the ins and outs of low-budget film industry. With “Slither,” Gunn splashes this experience all over the screen for his directing debut, making this horror comedy probably the most expensive film Troma never made.

Gunn is best known for two very depressing reasons: “Scooby-Doo” and its sequel. Yes, he’s the guy who wrote the scene involving a farting contest between Shaggy and Scooby, which doesn’t instill the most confidence in this young filmmaker’s debut as a director.

“Slither” takes Gunn light years away from CG dogs and into the heart of small town terror, looking to ick out the audience with goopy special effects and the primal terror found in having otherworldly slugs try to crawl down your throat. Gunn isn’t the first man staking his claim to this premise, following both David Cronenberg’s “Shivers,” and more directly, Fred Dekker’s 1986 cult classic, “Night of the Creeps.” Gunn puts more spirit from the latter into his film, hoping to hear both screams and laughs from the audience.

The screaming part of the movie doesn’t take much. Gunn has brought in some heavy make-up and CG artillery, with every wound, tentacle, and creature covered in slime. While the slugs are mostly CG organisms, the rest of the film is a make-up extravaganza, with emphasis placed on wet, grimy close-ups. There is some terrific work here.

The comedy portion of “Slither” is open for debate. Gunn has written some awfully hackneyed small town hick characters with little attention to detail, and fed them lines mostly consisting of cursing in progressively more unimaginative ways. It’s really par for the course for the genre, but as the film grows bigger, the characters grow thinner, leaving no one to root for, or (with a film like this) no one to root heartily for their death.

Gunn’s biggest mistake is not doing enough with the midsection of his film. Here’s the time when the film should throw down and start to give the audience thrill ride sequences the genre has subsisted on for decades. Gunn is so busy trying to arrange the story, and then pay it off with an unusual amount of plot detail and elaborate creature effects, that he forgets the joyride angle to the whole enterprise. “Slither” is ceaseless in its entertainment value, but it never breaks out of its tight cage and rampages across the screen, which this kind of madcap story deserves.

Rating: B