Amy Gentry’s debut novel Good as Gone opens with a universal nightmare come to life: in a nice Houston neighborhood, the safe kind, with sparkling pools and manicured lawns, 13-year-old Julie Whitaker is kidnapped at knifepoint in the middle of the night. Her ten-year-old sister, sobbing in the closet, is the only witness; their parents, Tom and Anna, blissfully unknowing, about to lose everything, lie asleep in the next room. And then, eight years later—after the billboards dedicated to finding Julie have been torn down, after Anna has forced herself to accept the unacceptable fact of Julie’s certain death—a familiar young woman appears, ashen and barefoot, at their front door. “And just like that, the worst unhappens. Julie is home.”

Unless she isn’t. Slowly, the doubts creep in. Details don’t add up. Omissions mount into lies. Secretly, silently, Anna is consumed by a disturbing thought: what if this “Julie” isn’t really Julie? “To me, the stakes of the domestic thriller are in that question,” Gentry says. “That moment of looking at someone that should be familiar to you, and then suddenly seeing a stranger there.” It is the horror that is at the crux of the novel: what if the people we know best in the world aren’t who we think they are?

It’s an idea that Gentry’s been playing with for more than a decade, loosely—very loosely—sparked by the case of Elizabeth Smart, who, in 2002, was kidnapped (also at knife point) and held for nine months by a religious fanatic would-be cult leader before she was rescued by police. “I started pecking away at it,” she tells me from her home in Austin, Texas. “What if a young woman came back after having been gone for a much longer time, and her mother didn’t know whether it was her daughter or not, whether she was an imposter.” Then, Gentry tabled the project. “I worked on it a little bit, and then did other things with my life besides writing novels, like going to grad school,” she laughs. (She has a Ph.D in English.)

Still, she kept the premise percolating. Since the beginning, Gentry had a sense of Anna—an academic, a pragmatist. The Julie character, though—dreamy and inconsistent—stayed just out of reach. “I almost felt that the character was concealing herself from me, as cheesy as that sounds. I felt like she was a liar. That’s almost the only thing I knew about her going into it,” Gentry says. “I knew what she looked like, and I knew she was a liar. So how could I believe this character?”

The key was in the structure: once she got the idea to alternate the two women’s stories, moving between Anna and the woman claiming to be Julie, the book “started to get really interesting to me as something I could actually write.” Anna’s story starts in the present and moves forward. “Julie” starts in the present and moves back through the past.

The alternating perspectives are more than a clever plot device. “I began to be really interested in writing about trauma and thinking about trauma,” Gentry tells me. “Maybe it’s just years of therapy, but there is this delving into the past that we associate with trauma—the idea that we’re all damaged people, and as you figure out why you are like you are, you’re peeling back layers of things.” Trauma has a way of reordering time.

Despite her longtime personal interest in the case, Gentry had mostly avoided the specifics, collecting details and “kind of putting them away.” She’d kept the People magazine cover story. She hadn’t read it. “When it was time to write, I thought, ‘Well, I should do research, I should go back and find out what her deal is,’ ” she recalls. But make no mistake: Good as Gone is hardly a fictionalized telling of real events.

One detail that Gentry noticed from the interviews Smart gave is her tremendous inner strength. “Even in the worst times, she really held onto her sense of self and didn’t ever believe that this”—the assaults, the attempted brainwashing—“was right.” But, Gentry wondered, “what if a person, for whatever reason, just didn’t have that feeling of an inner core to hold onto? Maybe that was [Smart’s] defining personality trait that gave her strength to cope, but what if it wasn’t?” The Julie character is unreliable, untrustworthy, inconsistent—an imperfect victim. She is not always easy to sympathize with. Which is the point.

“Victims of sexual assault don’t behave perfectly. They just don’t,” says Gentry, who has also spent time as an advocate for women who’ve experienced sexual violence. “There’s various reasons for that, but first and foremost, other than that they’re just human, is that they’re dealing with intense trauma, and their bodies are processing an incredible amount of shock and trauma.” The girl at the door can be a victim and a liar at once.

“It’s always been in the crime fiction form to investigate questions of identity,” observes Gentry. “Even detective stories, they’re all epistemological interrogations.” At its most basic, Good as Gone is about the impossibility of knowing even the people we treasure most. “At bottom, who really knows who they are?” she asks. “Maybe not even them.”