“Slither” might make for a fine popcorn film, if only it weren’t capable of destroying every appetite within a three-mile radius. Purely in terms of its gross-out factor, James Gunn’s directorial debut packs a powerful punch: Just about every disgusting scenario is played out on-screen with a ruthless candor that is partly horrifying, partly hilarious and mostly unnecessary. Then again, this kind of approach is exactly what makes “Slither” as thrilling as a roller coaster ride, and about as cerebral as one too.

One is never completely sure whether Gunn’s film is making earnest efforts to be a legitimate horror film in its own right or if it is just appropriating the usual tricks of other films in order to poke fun at them. The premise of “Slither” is so outlandish and the plot so formulaic that either possibility could easily be the case.

Grant Grant (Michael Rooker) is a local big man in Wheelsy, S.C., whose wealth outstrips his charm — and attractiveness — by a considerable margin. He is also married to a beautiful young girl named Starla (Elizabeth Banks) whom he loves dearly, at least in the physical sense. When he wanders into the woods one evening after carousing at a nearby bar, he stumbles across a gooey pod from a recent meteorite landing. Out of the pod ejects a spiky, wormlike creature that burrows into Grant’s stomach and nests in his brain, delivering his body and mind into the control of a sinister alien life form bent on — what else — planetary domination.

Over the next few days, Grant begins to morph into a gigantic monster that resembles a mollusk, a worm and Jabba the Hutt in roughly equal proportion. The boys at the Wheelsy Police Department — stereotypical yokels with a sizeable arsenal of assault rifles — set out to stop Grant when they receive numerous reports of a giant squid mutilating local pets and livestock. But Grant isn’t harvesting all that raw meat for himself — it’s also for the helpless floozy Brenda Gutierrez (Brenda James) whom Grant has impregnated with his tentacles and who swells up like Violet Beauregard from carrying thousands of alien worms in her stomach.

When Brenda bursts wide open, all hell sets loose on Wheelsy and it’s up to Starla and Wheelsy policeman Bill Pardy (Nathan Fillion) to defeat the worms (which invade people’s bodies by crawling into their mouths). It really doesn’t seem to matter how the conflict finally resolves itself, or even if the whole ordeal makes any sense in the first place, so long as Gunn can work as many revolting shots into the mix as he can. And so a man gets chopped in half by one of Grant’s tentacles, several zombie-citizens feast on raw human flesh, and Starla very nearly gets drawn into a love scene with her slimy slug of a husband.

Of course, it’s the locals themselves who provide the lion’s share of the fun. “Slither” is set in an exaggerated Southern town where the high school team is called the Cooters, where everybody seems to be packing a gun and where the biggest annual event is a hunting hootenanny known as Deer Cheer. In true Wheelsy form, minutes after making a high-speed car escape from a pack of aliens, the mayor (Gregg Henry) starts a temper tantrum in the backseat because his cooler is out of Mr. Pibb.

Still, even after all the mercilessly gory deaths, and even after lines like “The worms are in their brains!” it feels a bit like “Slither” squanders its potential. The dialogue could have been more inventive, the characters more colorful and the situations less derivative. After all, if one is setting out to make a cheesy, overwrought horror film, it seems only appropriate to pursue absurdity to the utmost, a-la “Mars Attacks” or “Shaun of the Dead.”

Instead, “Slither” spends most of its time straddling the line between a serious endeavor and an outright spoof, without ever settling comfortably into either category. Not that Gunn’s film isn’t clever in many places, or that it lacks its fair share of gleeful gore. Probably the best way to enjoy it is just to sit back, relax and sink into a prolonged zombie-like stupor.