“If we don’t split now, it’ll be much harder later.” How many relationships reach that ledge, where one person says something to that effect to their partner? Many, that’s how many. Too many to quantify.

In “Together,” the droll, fiercely well-acted codependency horror movie, Alison Brie delivers the news to Dave Franco, when their characters, Millie and Tim, a put-together schoolteacher and a coming-apart musician, have gotten a little too close in tricky, arguably unhealthy ways. But is it too late? Are they stuck with each other?

The Australian writer-director Michael Shanks makes his feature debut here, and for a movie so ardently devoted to body horror — a literalization of this couple’s particular dynamic — it has an unusually sprightly sense of humor. Brie and Franco, as you may know, are a married couple, and “Together” uses their ease with each other’s bodies and verbal rhythms to highly useful ends. Even when Shanks hits the primary theme of his movie a little too insistently, the actors are vivid throughout. Brie, especially, is spectacularly effective in every emotional register, in the keys of D (Distress), E (Eh what’s going on with our suction-lips?) and C (Commitment is all).

After a murky, fragmented prologue indicating another couple’s recent disappearance, “Together” gets down to its cautionary tale of city folk giving the country a try. Like “Se7en” and various other genre thrillers, this one’s intentionally dislocated so that it could be any big city — though probably New York, or possibly London— and any charming little town a couple of hours away. (The movie was filmed near Melbourne and Victoria, Australia.)

At a going-away party for Millie, whose new teaching job requires her and Tim’s move to a nice little burg on the train line, Tim suffers a mistimed and painfully public proposal of marriage. Millie is thrown for a loop, and the pause Millie takes before responding, two, maybe three seconds, lands like a day and a half. Brie is a wizard of timing and naturalistic cadence, and the film’s strategic introduction to their characters works like a flop-sweat charm.

After the move-in upstate, things seem initially better yet very quickly worse. The new house has an unexplained odor. Tim, insecure and itchy for a sense of career purpose, commits to a band tour, while Millie befriends a genial faculty colleague (Damon Herriman) who lives down the lane. The woods near Tim and Millie’s house are lovely, dark and deep, plus strange: Remnants of a ruined chapel of some kind have settled into the mucky earth. More suddenly, these two fall down into a literal and metaphoric well of trouble, a hidden entrance to an underground cavern laden with secrets as well as a pool of clear water that looks safe enough to drink.

From there “Together” escalates in cannily paced fashion, thanks to director Shanks’ forward momentum and editor Sean Lahiff’s destabilizing visual rhythm. After their underground discovery the couple isn’t the same. The teacher down the lane offers a sympathetic ear and some insights to Millie, who confesses her doubts about Tim, who cannot leave her alone for long. The neighbor responds with advice from Plato’s “Symposium” and Aristophanes’ theory (as written by Plato) of two human beings completing each other, aka the “Jerry Maguire” principle. “Together” relies on much blood and some severing of body parts, true, but from one angle it’s a rom-com with an all’s-well capper that “Jerry Maguire” didn’t have the nerve to try.

Filmmaker and screenwriter Shanks goes a bit far with his completion-theory thesis, with the Spice Girls’ “2 Become 1” on the soundtrack and perhaps one too many examples of the physical extremes undergone by Millie and Tim. The effects, however, are pretty terrific, especially in the neighborhood of the eyeballs. I’ve probably said too much, but it’s in the elegantly wrought teaser trailer, and while “Together” has a very different authorial voice (male) than last year’s “The Substance” (female), “Together” tells its fantastical tale a lot more efficiently.

It works, I think, largely because Shanks has the guts to write a male protagonist (though Millie has the edge, on the page and in the performance) who is no hero, no villain, just a mass of garden-variety insecurities, all too reliant on his partner for a sense of direction. Maybe these times have made it easier for male filmmakers to lay off redemption arcs and stalwart heroics, and lean into chaos and the humor of despair.

But an actor always has the last word in a character’s life, and here the key actor is Brie. While the combination of Brie and Franco serves the story well, in nearly every moment they share on screen she’s the one who makes urgent sense, both dramatic and comic, of every new relationship obstacle. On paper, Millie’s doubts conveniently fade when the story requires as much, and that’s very much a product of the man who wrote and directed this film. On the screen, with Brie, you buy it. And unless your ick tolerance is low to low-medium, you’ll likely roll with the merry ick of “Together.”

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'TOGETHER'

3 stars (out of 4)

MPA rating: R (for violent or disturbing content, sexual content, graphic nudity, strong language, and brief drug content)

Running time: 1:42

How to watch: Now in theaters

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