Though it’s doubtful anyone can really consider something called “mumblecore” an actual genre, the assortment of low-budget filmmakers making talky, intimate films on handheld video cameras (and their fans) can take great pleasure in what festival-circuit stars the Duplass brothers have achieved in “Baghead.” Beyond its engaging effectiveness as a flirtatious comedy that veers suddenly — horribly — into grindhouse dread, this mash-up also is a knowing satire about the creative hubris that drives the scruffier depths of the show business food chain.

The Duplasses, writer-directors Jay and Mark, had a cult hit with their 2005 film “The Puffy Chair” (about a La-Z-Boy’s journey across America), which they toured to various festivals and college campuses. “Baghead” is partly a commentary on that experience, which found the young filmmakers looming as role models for all the aspiring Facebook auteurs they encountered on the road.

They spoof themselves in the form of Jett Garner (Jett Garner), a young, fatuous firebrand whose film “We Are Naked” climaxes in a mumblecore trope: full-frontal nudity.

Sitting in the audience are four actor-writer types, each with a different set of designs on each other. Matt (Ross Partridge) is the hunky alpha dog of the crew who has been dating Catherine (Elise Muller) off and on for years. Catherine, who is narcissistic and insecure enough to obsess over whether her derriere is an “8” or an “11,” is another Los Angeles archetype: the self-consciously hot blonde who is pushing 40.

So, of course, she’s threatened by Michelle (Greta Gerwig), the slender, impish new import from the Midwest, who has her eye on Matt, even though she appears to be best friends with Chad (Steve Zissis), the chubby, nebbishy funny guy who is Matt’s best friend — and is truly, madly, deeply infatuated with Michelle, who is milking as much ego-boosting attention out of him as she can.

After a semi-successful effort to crash a post-screening festival party, the gang decides to go off to a forest cabin for the weekend and write its own movie to star in, although the ulterior motive is for Chad to try to hook up with Michelle.

In the time-honored slasher tradition of horny young people sequestered in the piney woods, the foursome sets up at their retreat and begins brainstorming. This mostly amounts to drinking, which dispatches the lightweight Michelle to an early bed. Sometime in the middle of the night, she wakes up with the spins and runs outside to throw up. Then she sees something in the darkness: Baghead.

Yes, a dude with a paper bag over his head, with little cut-out holes for eyes. She freaks. But when she wakes up in the morning, it all seems like a bad dream. Or was it?

This is where it really gets good. Matt decides to structure the group’s screenplay around the “Baghead” theme, even as everyone takes turns accusing everyone else of being Baghead when Michelle sees the mysterious figure a second time: stalking her in her bedroom. She mistakenly believes it’s Matt, picking up on a come-and-get-me invitation. Then she thinks it’s a jealous Catherine. Or could it be a jilted Chad? And so on.

The real fun is how the Duplasses manage the horror movie business as if it’s a poker hand, creating tension that magnifies the quirks and emotional prickliness of the characters, whose mutual neediness fluctuates with the love/hate dynamic of a reality TV competition (or college dorm room).

When all hell finally breaks loose, it’s that much scarier and that much funnier, mostly because you’re not sure whether to scream or laugh.

No spoilers here, but suffice to say that, for a few minutes at least, “Baghead” suggests the only proper season finale to any obnoxious TV relationship dramedy you can name.

Now busy in Hollywood, the filmmakers have a smart, incisive way with shaggy-dog chamber comedy. They won’t be satirizing their marginal status for long.