The sometimes-excruciating existential pain of “coupling” becomes physical torture — played-for-laughs — in the darkly comical “Together,” a bloody “body horror” rom-com about commitment.

Writer-director Michael Shanks, making his feature directing debut, pairs up Allison Brie and Dave Franco as a grounded teacher and a too-old-to-rock’n roll commitment-phobic man-child and puts them through a terminal, supernatural test that would drive any couple apart — surgically.

Millie and Tim are New Yorkers who leave “the city” so she can take a job teaching at a rural upstate middle school. He’s clinging to New York and his rock dreams, putting off facing reality as long as he can, taking what gigs come his way, anything to keep a foothold in “the city.” She’s clinging in other ways.

And when she proposes in the middle of their NYC going away party, Tim’s hesitation have her girlfriends clucking and Millie gritting her teeth. No, they “haven’t been on the same page for a while.” She dangles the prospect of them splitting up to him. He grasps for an apology.

“I f—–g suck!”

The older house they move into smells, and Tim proves useful in instantly tracking that stink down. His “dad’s toolbox?” With all the cutting tools? That’s “Foreshadowing 201” in film school, kids.

Because all it takes is a hike gone wrong, a tumble into a subterranean temple of some sort, with a root-tangled horror-movie pool straight out of “Alien.” As we’ve seen search dogs drop into this same spot, with grisly results, in the story’s prologue, we have an idea of what’s coming.

Things start happening, first to him, then to her. There’s “the pull, the thirst in our bones” that seems to be drawing them together. Literally. Lying together gets them “stuck” together. And sex? Try not to imagine what you know they’re going to experience and the movie is going to show you about that “uncoupling.”

Shanks sets the table for his leads to swap brittle digs, a couple who switch places in who’s clinging to whom. Little resentments grow into schisms, but whatever’s happening to them leaves no safe margin for that. They’re “Together” unless they take drastic, physical “body horror” steps to separate.

This is all depicted in dark and delicious eyes-averting detail.

Brie and Franco know how to find their way from grim to funny. The laughs come in their deadpan underreactions and freaked-out over-reactions at their plight.

It’s too bad it all turns rather conventional in the third act, with lots of unnecessary “explaining” and the like.

But “Together” is a smart, intimate horror picture in a year that’s seen a few of those, and the horror audience staying home for the most part. If you’re a fan, this is one you want to see in a theater. Because nobody should see and laugh at the grim goings on in “Together” alone.

Rating: R, bloody images, violence, painfully explicit sex, profanity

Cast: Allison Brie, Dave Franco, Mia Morrissey and Damon Herriman

Credits: Scripted and directed by Michael Shanks. A Neon release.

Running time: 1:42