The title psycho serves children demonic ice cream that turns them into zombie slashers. But it's a one-joke piece of shock theater.

Horror-movie directors are always looking for new ways to transgress. Eli Roth is a prime example — he once transgressed brilliantly, in “Hostel: Part II” (an eerily plausible movie about members of the elite paying huge sums to murder people in a warehouse in Slovakia), and he did it in an impish way in “Thanksgiving,” the holiday-horror expansion of his “Grindhouse” trailer. But with “Ice Cream Man,” Roth tries to go “over the line” in a way that’s shamelessly derivative, jaw-droppingly bare-bones basic as filmmaking, and way too ludicrous to scare you. The movie wants to be a lark, but it’s such a preeningly garish and ham-handed piece of shock theater that it makes “Thanksgiving” look like “Vampyr.”

In the picturesque town of Bayleen Bay, a sunny Little League game is followed by the arrival of the ice-cream man (Ari Millen), who drives up in a blue-and-white truck heralded by his jingling-bell theme music (which sounds like Eddie Cochran’s “C’mon Everybody”). All the kids rush over to get ice cream. But this ice-cream man is a sinister psycho creep. In his pristine 1950s ice-cream-parlor hat and suit, flashing a grin under a sardine mustache, he’s mute, like Art the Clown in the “Terrifier” films, and he gives off the pervy vibe of the shoe-store clerk in ZZ Top’s “Legs” video.

Is he a mad killer? No, he’s a pied piper, handing out demonically possessed ice-cream cones (the flavors advertised on his truck have names like Locust Lick and Serpent Swirl) that turn the local middle schoolers into slasher zombies who kill every teacher and parent in sight. It’s a “Village of the Damned” meets “Weapons” meets the opening scene of “Halloween” situation. But those movies all tower over “Ice Cream Man,” a one-joke piece of hyped-up kiddie zombie splatter schlock.

We know the kids have become a killer cult when they start skulking around with blue and chocolate ice-cream smears (with sprinkles!) around their mouths, as if they had just barfed up a double-scoop cone. Then they go on the attack. The first murder is a girl slicing into her slumbering mother’s neck with a hacksaw; then her brother pounds his father’s face in with a baseball bat. Roth himself plays the hero’s geek dad, who will get his just desserts. As the rampage ramps up, assorted tools and weapons are employed: an electric sander, a garden shears, a garbage disposal, a fishbowl, a baggie, bricks tossed from a rooftop — Roth’s slaughteriffic imagination is very analog suburban. And let’s not forget that old favorite: falling over backwards on a ladder and getting your face reduced to jelly by having it bashed in with a fire extinguisher. At one point the ice-cream man hands a girl a scoop, which juts out a blade, which she uses to saw the top of a teacher’s head off and then scoop out his brains and feed it to him. Fun!

I share the nostalgic affection, and even admiration, that people have these days for practical effects, but there’s a way that the reverence for them can be overstated. The use of practical effects in “The Odyssey”  is talked about in awed tones, as if it made up for the film’s storytelling lapses, and in “Ice Cream Man” Eli Roth’s obsession with decapitations and disembowelings and severed body parts — some of this is done practically, some digitally — is a thin step up from the carny-barker grisliness of Herschell Gordon Lewis. At the film’s frenzied climax, children jump rope on the playground using an intestine, and they play tetherball with a chain attached to a severed head. I’ve praised the “Terrifier” films for what may sound like the same thing, but the director of those movies, Damien Leone, uses practical effects to create escalating arias of gore. And Art the Clown’s mime-of-mayhem harlequin ghoulishness is part of the extremity. Since kids aren’t naturally aggressive in that way, seeing them become insane slashers is off-putting, in part because it feels so fake.

What “Ice Cream Man” comes down to, after a while, is an army of teen actors glowering in zombie ice-cream-barf makeup. The idiot school teachers keep getting it, which gives the movie a musty we’re-not-gonna-take-it “anti-establishment” vibe that’s very 1980s. And the film’s cheesy, functional, overbright look just feeds right into that.

There are three kids who aren’t infected, led by Jared (Charlie Zeltzer), the shrimpy geek who’s lactose intolerant (he can’t have ice cream). They escape the madness just in time to land in a freezer, where a dour priest (Benjamin Byron Davis) explains the ice-cream man’s backstory. It has something to do with a Catholic child-sex-abuse scandal and his being unfairly blamed for it, an origin saga so preposterous it would make Freddy Krueger groan. (My favorite part: the ice-cream man showing up in court…in his ice-cream-man suit.) That the film knows it’s preposterous is supposed to be redeeming. But Roth’s it’s-all-a-big-joke attitude here is a cop-out that’s beneath him, a way to avoid making a horror movie that truly engages us. With a slight nudge, “Ice Cream Man” could be a “Saturday Night Live” horror trailer. The Grindhouse might turn it down.