To say Naomi Watts is the only star in “The Desperate Hour” is a little misleading. She’s pretty much the only person in the film, that’s true. But you could make the case that she has a co-star in her iPhone.

Watts spends most of “The Desperate Hour” furiously typing, listening, consulting, pleading or otherwise glued to her smartphone as it takes on an outsized importance. Alone and lost in the woods, it’s her only lifeline to the world.

Watts plays Amy Carr as the first anniversary of her husband’s death approaches. She has a young daughter and a sullen, depressed teenage son. She goes on a five-mile jog into a forest and then finds out there’s been a shooting at the local school. The film’s meat is her harrowing hour of not knowing what’s happening in gut-twisting worry.

As an acting exercise, it’s intriguing — barely any scene partners and all unfolding in real time. As a film, not so much: After a plodding, placid start, it goes from first gear into fifth and never relents as the woes pile on.

It turns out watching an hour of Watts breathing heavily as she runs and gets more frantic, her eyes ever widening, isn’t that much fun. At one point, Watts’ character rolls her ankle, forcing her to limp the rest of the way. You might roll your eyes. Method limping.

Director Phillip Noyce squeezes out every possible trick to keep us interested. There are high, overhead shots of Watts running through colorful trees and shots of her looking upward from the dirt path. The camera sometimes swirls around her face like a wasp and ominous music swells.

In some ways, “The Desperate Hour” could be seen like an 84-minute commercial for the iPhone, as our heroine flips from FaceTime to live TV, one-touch Contacts, listens to music through earbuds, opens the Lyft or Instagram apps, checks locations on her satellite map, or relies on Siri’s robotic calmness. (“Starting route. Take Rosewood Trail for half a mile,” it chirps.)

But, in other ways, the film is as anti-technology as you can get, revealing that the apparent order promised by those rows of colorful apps on your home screen is just an illusion. Unwanted calls interrupt, connections are dropped, GPS can fail, live TV needs buffering, voicemail boxes get filled and, sometimes, simply no one answers.

Watts at one point screams in pure frustration — a feeling anyone who’s ever owned a phone will recognize. “Please stay on the line to speak to our next available operator,” says one of the recordings that sets her over the edge. In another scene, those three dots signaling someone is writing a text maddingly disappear.

The crisis reveals to Carr people who are really friends. A random car mechanic proves heroic, but a close pal going through the same crisis reveals a lack of empathy. The 911 dispatchers are amazing, maybe a little too amazing. “I might just be a voice on the other end of this line, but I’m here for you,” one says. “You did what any other mom would do.”

Carr might be lost in the woods, but any viewer by the halfway mark will know exactly where Chris Sparling’s increasingly melodramatic script is headed. We don’t need GPS to know that her family will be connected to the shooting and a ham-fisted attempt to speak about gun violence will be made. The moment she starts playing detective out there in the woods pushes the film past belief.

It’s a movie that could only be made now, with us dependent on our smartphones but they not always proving to be as smart as we’d like. Maybe that all changes with 5G, but here, there are long scenes in which a sweaty, panicked lady is just poking at her phone and swearing like a sailor. How her battery — or any viewer — lasts until the end is a mystery.

“The Desperate Hour,” a Vertical Entertainment release that hits theaters and on demand Friday, is rated PG-13 for “thematic content and some strong language.” Running time: 84 minutes. One star out of four.