One of the most tension-filled films of the year, “If I Had Legs I’d Kick You,” debuted on Friday at the renowned Sundance Film Festival in Park City, Utah.

Writer-director Mary Bronstein’s absorbing psychological drama about a mother at her breaking point is two hours of mounting anxiety and nervousness. 

For Rose Byrne’s falling-apart Linda, and for us.

I’m sweating just thinking about it.

Bronstein takes otherwise unremarkable domestic moments — driving with a pet hamster in the backseat, arguing with a parking attendant, dropping a pizza box — and depicts them through the eyes of a mom struggling to cope. Basically, a horror film. 

Every step and breath, casual or frantic, is laced with dread. 

Usually you shield your eyes when the serial killer’s broken into the house. Here, I was terrified when Linda took a walk around the block. 

Byrne, who audiences tend to associate with comedies such as “Bridesmaids” and “Neighbors,” gives a career-best performance as Linda, the fraying parent of a young daughter who eats via feeding tube because of a disorder. 

As the pressures pile on, the actress is so unrelentingly intense you can practically feel her molecules vibrate.  

Adding to the grind, Linda’s impatient husband is away on business, and she has to work at her therapy practice during the day and take care of her child at night. 

When she’s not guzzling whole bottles of wine at 4 a.m. 

The little girl’s meal routine is a demanding exercise that wakes Linda up in the middle of the night to refill a pouch. Her daughter needs to weigh at least 50 pounds inside of a week or else, a medical professional threateningly tells her, “we’ll need to reassess care.”

Then, amid her family woes, the ceiling of her bedroom collapses, leaving a cavernous hole, and Linda and the kid are forced to stay at a dingy motel down the road. 

Bronstein’s film, darkly but colorfully lit, verges on the supernatural at times. That gorge above Linda’s head takes on a spooky magical-realism quality. Spaceship light emanates from it, as well as firefly-like beings. Without ever becoming pretentious, we get the sense that the void is also Linda’s own.

“If I Had Legs” is not “Spy,” but the actress doesn’t totally toss aside her comedic chops either. As Linda’s thrown caution to the wind, her reactions can be hilarious. Those few laughs are big because they are a brief reprieve from the near-constant anxiety we’re feeling.

Someone who needed to rein in the funny is Conan O’Brien, who makes his feature acting debut in the film playing Linda’s colleague and therapist she’s infatuated with. This year’s Oscars host is quite good as a cold, distant, un-empathizing jerk.

But don’t go telling him that. Onstage after the premiere, O’Brien reviewed himself.

“I feel like a complete fraud,” he said.

And A$AP Rocky plays her motel neighbor, who’s both supportive and an enabler. 

Most haunting is Danielle Macdonald (“Dumplin’”) as Caroline, Linda’s paranoid patient and, perhaps, a vision of what Linda could herself become in she doesn’t fix things.

Bronstein’s film is a tough watch. There is archival footage of Andrea Yates, the woman who drowned her five children in a bathtub, and Yoselyn Ortega, the nanny who stabbed two New York kids. 

And there are some grotesque moments involving the human body and animals. 

If you can stomach that, the jitters, increased heart and outright terror are worth it for Byrne. She’s unforgettable.