What does it mean to love a child who doesn’t belong to you? This is the central question of “Stranger Care,” and by asking it, author Sarah Sentilles shines a light — or beams a heat lamp — on all the ways there are to love, be a parent and experience loss.

In prose so gripping it reads like a thriller, Sentilles describes the choices that led to the moment when she and her husband are on the phone with a social worker, saying yes to fostering a 3-day-old girl. She leads us through her 30-something ambivalence about children; her realization she wants a baby; her husband’s resistance; and their compromise to try to adopt through the foster care system.

While most foster parents get certified so they can care for the child of a relative or friend, Sentilles and her husband, Eric, are classified as “Stranger Care” — foster parents who will have no connection to the child they bring into their home. Their choice illuminates another truth: that society cannot survive unless strangers also care for the people we love.

When the call finally comes that there is an infant girl, Coco, who needs care, Sentilles and Eric drive to Twin Falls, Idaho, from their home in Hailey. What transforms them from strangers to parents in a moment is the vulnerability of the tiny baby who awaits them, the sense that they are needed, essential. Sentilles describes the experience of becoming a parent exactly. Your love for your child feels infinite, but what binds you to them is their infinite need, their helplessness.

That this love story feels doomed from the start is what makes this memoir so devastating. Sentilles and Eric are told that Idaho is a “reunification state” — i.e., family reunification is the state’s priority, and a biological parent only has to meet minimum requirements. And yet, like Sentilles, I kept hoping, down to my last atom, that Coco would stay with her. Which meant I was rooting against Coco’s biological mother, who also loves her, though it’s clear she’s not the better caregiver. This is an uncomfortable feeling, and Sentilles doesn’t let herself or anyone else off the hook for it.

Instead, she opens herself to loving a child she knows she may not be able to keep. In doing so, she experiences a more extreme form of the vulnerability that is universal to parenting — the fear of losing your child, of not being there for them. What Sentilles and Eric grapple with is not the abstract terror of loss, but the likelihood of it.

How do you parent in that space? How do you love in that space? Sentilles and Eric find a way, they throw themselves headlong into it, and this bravery is astonishing and humbling. What makes this book so powerful is that by experiencing motherhood through the lens of fostering, Sentilles is able to look at the wrenching and worn-out topics of parenting in a new way.

She also scorches the foster system, the random, bureaucratic terror whereby the social worker you get — one who is good at their job and cares or one who doesn’t — determines so much of the outcome for children and families, even in a system that is supposed to be governed by consistent rules. In the end, the state fails Coco — it fails her foster parents, it fails her biological mother, and it fails the child it’s designed to protect. How do you find any grace in such an ending?

Sentilles did not give birth to Coco, she shares no DNA with her, she has no legal claims on her, but this child made her a mother. She is a mother. There is grace in that.

“Stranger Care: A Memoir of Loving What Isn’t Ours”

By Sarah Sentilles

(Random House; 432 pages; $28)