A mother and daughter with everything they own in their SUV move from Tuscon to Oklahoma, and rue the day Mom takes a shortcut through Tulia, Texas.

That’s where her tire blows, literally a moment after her cell phone has let her know there’s “no service.” That’s where little Clara (Apollonia Pratt) wanders away from the car, just far enough to stumble into a “Rattlesnake.”

Yeah, we knew it was coming. From the TITLE. But it’s what comes after that fuels this nightmare and makes it a horror movie.

Because desperate Mom (Carmen Ejogo, who played Coretta Scott King in “Selma,” and was in “Fantastic Beasts and Where to Find Them”) seeks help in a battered trailer she spies just after the bite. The Woman (Debrianna Mansini) may look Okie Goth scary, but she lets Clara lie down while Mom, Katrina, frantically changes the tire.

Katrina picks up the kid, who looks better, and dashes to the hospital. The doctors see no problem. They start asking Katrina questions, about how tired she is, how stressed.

Hey! I KNOW what I saw!

And then “The Suit” shows up. Bruce Davis conveys “no discounts” hospital administrator menace as he brings up Katrina’s “debt.” And then he turns scary.

“I’m not talking about hospital debt…Her little soul was spared.”

Satanic yadda yadda yadda, “She will suffer,” and we and Katrina learn the “debt” is a-soul-for-a-soul thing.

So Katrina’s trip to the trailer was an actual Deal with the Devil. Only she didn’t know it. Now, she’s got to go out and find somebody to kill so that her little girl can live.

Maybe she doesn’t need to leave the hospital, she reasons. Or maybe she’s going to need a gun. They have those in Texas, right?

Here’s what doesn’t work about “Rattlesnake.” As much as one hesitates to ever truly call out the person in front of the camera for being a movie’s reason for failing, Ejogo is just plain off here.

We get no sense of Mom’s mania, any notion of rising desperation as the sun moves across the Southwestern sky and the ticking clock ticks down towards sunset.

The moral dilemma of standing over a dying old man in a hospital, just after you’ve pulled the pillow out from under his head so that you can smother him with it? The ethically murky hunt for a victim, wondering how to answer the weaselly off-the-books gun dealer (David Yow) who offers her a Glock and asks, “Who’s the unlucky son of a bitch? He have it coming?”

Ejogo gives these moments all the fraught emotion of a mother in the market aisle, trying to decide between Peter Pan or Jif.

Writer-director Zak Hilditch serves up a gritty setting, and maybe the funniest continuity error I’ve ever seen in one of these quick-and-dirty “Netflix Originals.” A bullet riddled trucker chases Katrina swinging a tire iron, and when he stops of shoulder it — it’s an adjustable wrench instead.

There are also a couple of harrowing encounters with the Dead whose spectres now monitor Katrina’s progress, amping up the threats and even assaulting her — such as the dead tween who beats his head on her SUV window until it shatters and Katrina tumbles into the street and into the path of an oncoming truck, an assault only Katrina sees.

But Katrina never lets us feel the panic or appreciate the stakes, here. And whatever the director (he did the Netflix Stephen King adaptation, “1922”) didn’t push for in the performance, it’s the acting that lets the picture down most of all. And that’s all on Ejogo.

MPAA Rating: TV-MA, violence, profanity

Cast: Carmen Ejogo, Theo Rossi, Emma Greenwell, Bruce Davis

Credits: Written and directed by Zak Hilditch.  A Netflix release.’

Running time: 1:26