Gin Phillips had no intention of writing summer’s most scintillating literary thriller.

“I wanted to write a book about motherhood,” Phillips says of Fierce Kingdom, the story of a mother and four-year-old son trapped in their local zoo by an active shooter event.

“I was aware it was a faster plot than I normally focus on,” she says, “and I loved the idea of having to tell a story within such tight confines, where there’s literally geographic walls to it and a real set time period....But I never thought of the term ‘thriller’ until editors started using it.”

Fierce Kingdom is an artful meditation on motherhood and humanity that’s timely, tense, and propulsive. “Poignant and profound, this adrenaline-fueled thriller will shatter readers like a bullet through bone,” Kirkus writes in a starred review.

The zoo is decorated for Halloween and it’s almost closing time as Joan and her son, Lincoln, make their way to the main exit. When she she sees a man with a gun—realizing the human forms on the pavement aren’t blown-over scarecrows—she grabs her boy and runs like hell.

Where to run? Where to hide? What to do if they’re discovered?

“She had thought this in no more than ten footsteps,” Phillips writes, “so quickly and so slowly...and all this thinking is getting her nowhere. The lion roars, from a distance, and it is not a shocking sound, because they feed the animals right before closing, and the lion is always vocal, anticipating. It roars again, comforting almost. She is surrounded by wild things in boxes. She feels a thrum of solidarity.”

Phillips, who writes adult and middle-grade fiction, is the author of five books including the 2009 Barnes & Noble Discover Award-winning novel The Well and the Mine. She lives in Birmingham, Alabama, with her husband and five-year-old son, Eli, to whom Fierce Kingdom is dedicated.

“One thing that certainly was in my head as I started writing the book,” she says, “and that Joan echoes on some level, is what a fascinating thing it is as your child starts to become a real person and not just an adorable blob—someone with a mind of his own, with reactions you could never predict.”

Joan is in awe of her son’s growth, his idiosyncratic reasoning. “[I]t is one of the bits of mothering that has delighted her all the most because she did not know it existed,” Phillips writes. “His mind is complicated and unique, weaving worlds of its own....He is a whole separate being, as real as she is.”

Though Joan can’t imagine her life without Lincoln, she does imagine what facing their present danger alone would change: how brave she’d be, the plans she’d devise, the people she’d save, the risks she’d take to escape.

“She’s someone who wants to do the right thing,” Phillips says of Joan, “but with her child next to her, what she’s willing to risk is very different. “One of the big questions of the book is not just what would you do for your own child—which I think most people believe is just about anything—but what would you do for someone else’s child? What would you do for a stranger?

“That’s a more powerful question to me,” she says.

Megan Labrise writes “Field Notes” and features for Kirkus Reviews and is the co-host of the Kirkus podcast, Fully Booked.