What follows is an intensely perverse game of hide-and-seek, with Joan and her son finding sanctuary in spots of varying ingenuity. Joan’s cellphone becomes a lifeline, then a glowing bull’s-eye. The music still streaming over the loudspeakers is by turns reassuring and creepy. (A deliberate echo, I am guessing, of the Westgate Mall massacre in Nairobi, when canned music also played obscenely in the background as Shabab militants roamed the shops.) She meets other trapped visitors at the zoo, and we get guided tours through their harrowed minds.

In horror films, the girl always twists an ankle. That’s the cliché, her impediment to escape. For Joan, the main obstacle is a 4-year-old who cannot always remember to use his inside voice and whose stability is in inverse proportion to his blood sugar. Seldom are the banal logistics of child rearing — does Joan risk a trip to the vending machines to avert a hunger meltdown? how does she keep Lincoln occupied? — as riveting as they are in this book.

Phillips, the author of four previous novels, including two for middle-grade readers, beautifully captures the quirks, tedium and magic of parenting a young child. How you become a multitasking juggler fit for a circus. How you learn to speak fluent Superhero. How your mind becomes a time-lapse machine, fast-forwarding to the bittersweet moment when your kid will no longer helplessly depend on you. How your kid’s own mind is a surprise machine. Joan is always stunned by Lincoln’s. “But she will never really know it all, and that is the thrill,” Phillips writes. “He is a whole separate being, as real as she is.”

Lincoln’s mind also happens to be my one midsize complaint about “Fierce Kingdom.” Maybe some 4-year-olds know the word “intestine” and engage in linear reasoning and collect presidential trivia and ask why people say “good grief” instead of “bad grief,” but Lincoln’s unrelenting cleverness becomes grating. (I half-wondered whether Phillips had to create a tiny whiz because a regular 4-year-old would eventually become a monotonous sidekick.)

But before Lincoln’s chatter is grating, it is endearing, and we never find him grating as a character; we fret about his safety from start to finish. Joan’s desire to protect him is total, feral. (Not an accident, probably. We are in a zoo, where the maternal sheltering instinct is regularly on display.) “Fierce Kingdom” is a diabolical enactment of a mother’s most tortured and catastrophic thoughts.

This may be what gives Phillips’s thriller its true power. Disaster fantasies are familiar floats in every parent’s midnight parade of horribles. So it is with Joan. From the moment Lincoln was born, she has felt intimations of dread. She’d watch her sister-in-law strap him into her car and imagine them crashing on the Interstate. She’d drop him off at school and suppress visions of a shooting, “men pushing their way into classrooms.”

And now she and her son have become human quarry in a zoo.

As “Fierce Kingdom” progresses, we watch Joan go through what I came to think of (informally) as the five stages of terror: panic, lucidity, fury, calm, bravery. “Make yourself disappear,” she repeatedly tells her son when she needs him to be quiet and still. It is one of the most distressing things to have to say to a child — that sometimes help is not immediately forthcoming, that sometimes the terrible things that go bump in the night are real.