“War Pony” is a compelling, wholly-lived-in drama that tracks the dead-end lives of two aimless young males of the Pine Ridge Indian Reservation in South Dakota and Nebraska. We follow immature-even-for-19 Bill and tweenage Matho as they navigate their opportunity-deprived world of poverty, drugs and no parental guidance, each seemingly-destined to repeat the cycle that created them.

It’s moving and immersive experience with hints of “Trainspotting” in how it feeds our growing fear for who will pay the consequences for these awful decisions, and a taste of “Smoke Signals” and “Songs My Brothers Taught Me” in its depiction of life “as it is” “on t”he Rez.”

The film’s narrative aimlessness is in keeping with the life-paths of multiple baby-mamas Bill (Jojo Bapteise Whiting) and just-starting-to-go-wrong Matho (LaDainian Crazy Thunder) and its humorlessness keeps the viewer at a sort of clinical remove from the characters.

Left with a depiction of reactive, impulsive and immature decision-making by young people almost wholly on their own in the midst of a culture that has collapsed — with only pow wows for show and a nice school most of the boys check out of — the overriding feeling this film feeds is despair.

One of Bill’s toddler-sons is with him in his mother’s house. Mom is a new grandmother who wears the mileage of a great-grandmother, and hearing his “I ain’t got time for this s–t” when he hears why Carly, the child’s mother, isn’t around annoys us almost as much as his mother.

“You ain’t got time for what ‘s–t?’ Your baby mama, or me?”

Carly’s been arrested. Her threatening “man-the-f–k-up” calls to her ex all on deaf ears. Bill doesn’t have any cash. He’s already moved on to Echo (Jesse Schmockel) and has a little boy with her, too. Not that she wants anything to do with him. That ancient Chevy Caprice he drives? Stolen from her, we gather.

Matho is a distracted boy of eleven or twelve who’s hanging with three other free-range tweens, smoking, pilfering, hitting on girls and scavenging each others’ houses and single-wides for food.

His occasional stumble into his druggy/maybe-dealing dad has the kid shrugging off “the last time you ate” questions. Not that we figure the two-fisted father-figure gives a damn.

Stealing Dad’s stash and making those first meth-pills sales earns Matho and his boys junk food to get them through another day. School? That’s for passing notes to girls and maybe acquiring that one sure-thing meal a day at lunch.

Bill’s job-hunting is half-assed, his eye for “opportunity” a bit juvenile. He finds a woman’s “poodle with papers,” returns her and resolves to raise the cash to (overpay) for the dog and breed it. Because poodles are a favorite accessory of the 17,000 or so Oglala Lakota Sioux who live there?

It’s while raising cash for the dog that he helps a white Nebraska rancher (Sprague Hollander) and haggles his way into a dubious “help around the place” job that includes drinking wine with Tim and his lonely wife (Ashley Shelton) hustling turkey-farmer/processor Tim’s assorted “girls” back to the Reservation after he’s enjoyed their company.

The poodle is impregnated, he has cash to “help out” Echo and their baby, and his mother and the baby he had with the still-jailed/still fuming Carly. Bill’s making it, he figures.

Matho just gets drunk and stoned and beaten and kicked-out of the house when his father figures out he’s been stealing. Matho’s lifeline is a woman who takes in boys, a woman who turns out to be a drug dealer who oddly demands that he “keep your drama out of my teepee.” With Matho trying like hell to grow up too fast and bravado his way into “manhood,” fat chance that’ll happen.

Each guy’s tenuous chance of beating Pine Ridge’s nationally scandalized low-life-expectancy is sure to be tested.

Co-directors Gina Gammell and Riley Keough let the narrative drift from one lad to the other as they’re more interested in immersing us in the feel of this world and the logistics of this life. Their one artsy touch is having Bill stare down what we can guess is his spirit animal, a bison, a few times when he’s losing his way.

The hooting and hollering funeral caravan that makes up one scene is a Sioux spin on your standard American funeral, overloaded pickups weaving all over the highway in their procession. A bit more local color like that would have been appreciated.

The movie’s narrative and entertainment value attitudes seem summed up by Carly when she gets out of jail and Bill hits her with the “Mad at me?” baby-daddy puppy dog eyes that always seem to work for these dudes.

“Didn’t expect much from you.”

Whiting has a magnetic, young James Franco screen presence here, a bit dopey and lazy but somehow dangerous. Crazy Thunder’s Matho doesn’t meet Bill until the third act. But everything the younger kid does seems designed to emulate the older one and make Matho The Next Bill.

Its lack of star-power and depressingly downbeat story may limit this film’s prospects, even if Elvis’s granddaughter did have a hand in making it.

But the two Native actors-turned-screenwriters — Franklin Sioux Bob and Bill Reddy — whom Keough met while filming “American Honey” give “War Pony” an authenticity that is hard to beat. If it hurts a little and leaves you a tad hollowed-out by then end, that just means they got the details right.

Rating: R for drug and alcohol use involving minors throughout, pervasive language and some violence

Cast: Jojo Bapteise Whiting, LaDainian Crazy Thunder, Jesse Schmockel, Ashley Shelton, Sprague Hollander, Iona Red Bear

Credits: Directed by Gina Gammell and Riley Keough, scripted by Franklin Sioux Bob, Bill Reddy, Riley Keough and Gina Gammell. An eOne/Momentum Pictures release.

Running time: 1:55