Harper’s an LA professional woman in her late 20s, an ad campaign director as put-together, perfectly turned-out and organized as her pristine, perfectly-conceived magazine ad shoots.

And then, just as she gets the news that a prestigious account has landed on her desk, she gets a call from Child Protective Services in Las Vegas. A mother of two there has lost control of her life, with a teen raising a younger half-brother in a house with no food, no electricity and mom passed out on the couch.

We can’t make out Harper’s connection to all this, only her efforts to call her father, somewhere in the middle of the Pacific. Next thing we know, this organized, responsible woman is picking up that teenager from school. The kid’s coming back to Los Angeles with Harper, and that’s that.

The story of “No Right Way” could play out in a lot of scenarios, many of them variations on a “Baby Boom” theme. But writer, director and star Chelsea Bo had a hand in scripting an earlier “parenthood” riff, “Fully Realized Humans.” Expect anything but the expected.

Harper isn’t the sister of the manic, blame-everybody-else stoner mother (Eliza Coupe). Tiffany was merely another ex-wife of her father’s. Dad’s years of warning to “avoid that mess” might explain the years since she’s seen Georgie, her half-sister.

But despite that, in spite of her father’s sat-phone warnings to do nothing, avoid “that mess” again and despite her own tidy, careerist life, Harper is stepping up. No foster care for her sister, no ma’am.

Georgie half-complies. But she’s a sk8rgrl and not inclined to shower on a regular basis. She borrows Harper’s phone for giggly catch-up calls to all her Vegas friends, picks up and inspects every tchotchke in Harper’s house and lets on to a “free range” life of 13 year-old breaking-and-entering.

Whatever was going on at home, Georgie takes her mother’s side, parrots every excuse and accusation against Mom’s latest ex, Teddy, and vents her ongoing fury at being separated from her little (half) brother.

Harper serves Georgie healthy (vegan) food, says all the right things to bond and sets mild “boundaries” as she dips her toes in “parenting.” And being a rules-following, organized problem solver, she sets out to do something about “this situation.”

Our writer, director and star keeps everything mild-mannered, everything orderly and almost touchy-feely in this “big sister” enterprise, until the third act, when we see just how far out of her depth Ms. “Organized” with a “Plan” is and just what she’s up against back in Vegas, or way off in the disconnected Pacific.

Bo’s Harper is a solid, stable presence in this, leaving room for “American Horror Story” veteran Acres, “Happy Endings” alumna Coupe and no-nonsense “Veep” veteran Sufe Bradshaw, as the mother of Georgie’s best friends, to shine.

The acting is as spot-on as the pacing, a story that lures us in and charms up before delivering that cold slap of reality.

“No Right Way” is downright triggering in the ways it recreates every encounter a reasonable, responsible person has when dealing with the unreasonable. Not that Harper knows what she’s doing, but if you’ve ever had arguments with a rageaholic or a hysteric, you’ll sympathize with the idea that the irrational and ill-tempered are ganging up on the sanest person here.

And even the sanest among us can get in over our heads, misread the room or a family dynamic.

What Bo’s made here is a movie about parenting for people who aren’t parents, those of us who can see problems and propose solutions, but reach the limits of our experience and our suggestions for handling something like this a lot quicker than we’d think.

With statistics all over the world pointing to lower birthrates and longer delays in starting families, “No Right Way” feels timely and humbling as it sets our “That’s what I’d do” expectations up, and slaps them around all through the third act.

You can follow the rules and do what you’re sure is “right,” but when there’s “No Right Way,” the results are going to be unexpected, unsettling and damned unpleasant at times. And that’s just the way it is.

Rating: unrated, some violence, profanity

Cast: Chelsea Bo, Ava Acres, Eliza Coupe, Ty Cortes, Guy Noland and Sufe Bradshaw

Credits: Scripted and directed by Chelsea Bo. A Paxeros production.

Running time: 1:43