“The Mountain Story,” by Lori Lansens. Simon & Schuster, New York, 2015. 312 pages. $26.

Intertwining plotlines vie for urgency in Lori Lansens’ nail biter, “The Mountain Story.” And though her characters change or grow, astute plotting drives this intense story of three women and a suicidal young man who are strangers one moment and acutely interdependent the next.

The group members, each with serious baggage, find themselves lost on a remote, only partially mapped mountain in late fall. They are without food and water, and they are freezing. There’s no hope of rescue. The older woman suffers a life-threatening injury. And, as their conditions deteriorate, wild animals take notice and commence a relentless stalking.

Lansens is a first-rate storyteller. Author of a bestseller about conjoined twins, “The Girls,” Lansens is from Ontario and now lives in Santa Monica. She has a background in screenwriting and has also published the novels “Rush Home Road” and “The Wife’s Tale.”

Wolf Truly, the protagonist in “The Mountain Story,” writes a letter to his adult son that explains not just what happened for those five days on the mountain, but everything of significance that led up to it. This story, writes Wolf, is going to change your life forever. The mountain ordeal begins on Wolf’s 18th birthday, the day he decides to kill himself.

Wolf is right. The story is full of surprises that most readers won’t see coming. One thing Lansens is almost too good at is evoking squirms from the squeamish. Wolf’s father, Frankie, is a vile drunk who uses everyone including his son to get what he wants. His behavior is revolting.

Wolf’s beloved mother, a tempering agent for Frankie, dies early and her death is one of the plotlines readers will hang on. Frankie moves Wolf to the California desert with the promise of a better life. That’s a lie.

Instead Wolf winds up in Tin Town, a filthy collection of rotten trailers. Wolf sleeps on the floor with a bunch of dirty and unsupervised toddlers he must clean up after.

Wolf makes friends with Byrd, a young man close to his age and a bona fide soul mate. They click on every level and together explore the nearby mountain with growing confidence. Byrd, knowing and competent, is Wolf’s one link to life.

Without Byrd, Lansens convinces us, we can’t imagine how Wolf could live out the dismal existence he’s been handed. And, of course, Lansens gives us good reason to worry for Byrd.

When Wolf finds himself in the company of three women — grandmother, mother, daughter — who need his help finding their way to an unmapped mountain lake, he realizes he has no choice but to lead the way.

His own plan for that day, the day of his 18th birthday, is suicide. He brings nothing to the mountain because he no longer has need of anything. A series of plausible mishaps thrust the group deep into a hostile wilderness that, for very good reasons, they cannot escape.

Likable Nola, the sensible and compassionate grandmother, has come to the mountain to deposit her husband’s ashes on their wedding anniversary. Her daughter Bridget and granddaughter Vonn accompany her grudgingly.

When the four of them slide down a long and steep expanse of loose rock, the story turns dire in a hurry. They lose their small stash of food and water right off the bat. Nola’s arm breaks, bone pierces the skin and she quickly develops a foul infection. Vonn is mysteriously sick to her stomach. And Bridget is a hysteric whose behavior imperils everyone.
Lansens keeps all the plots rushing forward, with twists and turns that jolt us from habits of expectation and anticipation. Her nature setting reminds us how powerless and unprepared we are when left to our own devices. Resourcefulness and determination, two attributes the group possesses, cannot alone make the difference in survival.
Human vulnerability is a theme that never gets old. Lansens reminds us that although we consider ourselves masters of our universes, we are, in fact, utterly reliant on interdependence and simple good luck.

Rae Padilla Francoeur’s memoir, “Free Fall: A Late-in-Life Love Affair,” is available online or in some bookstores. Write her at rae.francoeur@gmail.com Read her blog at freefallrae.blogspot.com or follow her on Twitter at @RaeAF.