PARK CITY, Utah — The film that will have everyone buzzing after its premiere at this year’s Sundance Film Festival is “Twinless.”

Bring an oxygen tank, you’ll be gasping so much. And not always from laughter.

A Sundance debut was a shrewd move for writer-director James Sweeney’s totally unexpected dramedy about two 30-something men who bond over the fact that both of their twins have died. 

The storied cinema locale makes for a mischievous bait-and-switch. 

That’s because you stomp in assuming the flick starring Dylan O’Brien and Sweeney will be the familiar kin of countless other sweet, quirky indies, in which paralyzing grief gives way to hipster joy, that have bowed up here in the Utah mountains. Most of us can recite the plot from memory. There are probably vintage bicycles.

And doesn’t the quick synopsis sound lovely? Two urban people losing their biological other half and filling their seemingly unfillable void with against-type friendship? Even the title comes off like a nice ’90s VHS.

“Twinless” begins innocently enough, but soon transforms into something much darker and more psychologically sinister. Sweeney’s unpredictable movie twists and writhes, while never losing sight of its layered characters’ heartfelt longing. Still, don’t go for feel-good. Go for feel-shocked. Definitely go for feel-complicated.

Sweeney’s compulsively addictive film begins with the loud noise of car crash, and then quick-cuts to a funeral. This director revels in abrupt cuts, which deliver laughs as well as cringes and lend the film a brisk pace that never lets up.

Roman’s (O’Brien) brother Rocky is in the casket, and he and his mom (Lauren Graham) are overcome with sadness. The emotionally confused attendees fall apart when they see Roman, Rocky’s identical twin. They sob and hug him like their friend has come back to life.

“We’ve never met before, but I feel like I know you,” one says.

And whenever anybody on the street mixes him up for his late brother, Roman tends to lie. It’s just easier.

At a support group for grieving twins, he meets spindly, awkward Denis (Sweeney), whose brother Dean died a year and a half earlier. Denis is gay and chatty; Roman is a straight, athletic meathead, who blurts out malapropisms like, “I’m not the brightest knife in the drawer.” 

The unlikely duo hit it off, and become inseparable. Borderline obsessive. Cue the kindness, we think. Not so fast.

To give some idea of the evolving tone, during a party with a lot of simmering subtext, Sweeney splits like screen like Brian De Palma in “Carrie” or “Dressed to Kill.”

From here, the less you know, the better. Sweeney — a mega-talent who is as terrific an actor as he is a writer and director — is a brilliant plotter. Those who are always a mile ahead of a script will find themselves refreshingly behind this filmmaker’s razor-sharp mind.

He’s also smart to risk doing double-duty as Denis, a tricky character with a hyper-specific tone and brow-raising behavior. If handled clumsily, the the geeky friend could easily get a cool response from the viewer. Even at his worst, we give Sweeney’s endearing Denis the benefit of the doubt. Hard to admit (you’ll find out why), but I think we also all see ourselves in him.

O’Brien wears two hats, too. He also plays Rocky, Roman’s extraverted gay brother, in flashbacks from before the accident. Although the stark contrast between the siblings is impressive, the actor’s most shattering moment comes in a hotel when rough Roman uncages his trapped feelings. I can’t recall such a raw performance in a comedy in ages.

You’ll begin “Twinless” with basic expectations, and you’ll end it with your mouth agape.

And then you’ll ask the most satisfying question there is after first encountering an exciting young filmmaker’s work: When’s the next one?