“Our immigration system is broken,” Barack Obama said in November 2014. “And everybody knows it.” His administration would henceforth, he announced, focus its enforcement efforts on “felons, not families. Criminals, not children. Gang members, not a mom who’s working hard to provide for her kids.” When the Trump administration triggered a series of raids to round up undocumented immigrants two years and some months later, the “not families” promise was gone.

For topicality, Lisa Ko’s novel The Leavers, about an undocumented mother who suddenly disappears and the young American-born son she leaves behind, could hardly be better timed. The political resonance of The Leavers is no coincidence; Ko got the idea for her affecting debut from a 2009 New York Times article about an undocumented immigrant from Fuzhou, China, who spent a year and a half in detention (much of it solitary) after being arrested at a Greyhound station in Florida on her way to a new job. That woman’s story inspired the character of Polly Guo, the mother in Ko’s book; her son, also mentioned in the article, yielded Deming, Polly’s 11-year-old.

These days, it’s difficult to imagine a real-life Deming who is not, by age 11, excruciatingly aware of the perils of having his sole parent be a poor immigrant of ambiguous legal status. But Ko has made the notable choice to render Deming almost totally ignorant of those perils. When Polly fails to come home from work one day, he’s told she’s “visiting friends,” and though he knows “she had no friends to visit,” the possibility of her absence being immigration-related never occurs to him. He worries vaguely that “she’s in danger,” but envisions a scenario in which “she’d been the victim of a crime, like on CSI, and maybe she was dead” rather than wondering if perhaps she’s being punished for a crime instead. Knowing Polly had fantasized about taking a job in Florida, Deming concludes that “she’d left for Florida and left him, too.”

Readers are, like Deming, propelled forward in search of Polly, the cause of whose vanishing remains a mystery (unless you’ve read the back cover). We pick up clues along the way about her turbulent past, her treacherous emigration from Fuzhou, her hopes for the future, and her hasty and unexplained departure from the life in New York she’d worked so tirelessly to build. Intertwining two familiar narratives—the struggling immigrant saga and the lost-child tale—Ko homes in on the latter. Deming’s perspective is in the foreground as he weathers the trauma of relocating to a small upstate town and rebuilding his life as “Daniel,” the foster son of a well-meaning, if clueless, white couple.

In focusing on a bewildered young victim, The Leavers follows a convention of the protest novel genre; Ko dramatizes the personal—a family torn apart—in order to draw attention to a structural social problem. And Deming’s utter ignorance of that social problem looks like an inspired way around the sentimentality and thudding moralism that haunt the genre. Deming’s side of the story could easily have been dominated by a heavy-handed sense of despair about the immigration system’s injustices. Instead, in his mind, he’s a child who has lost a parent. Politics aside, Ko implies, that’s all that should matter.

Somehow, though, Ko’s choice doesn’t quite rescue her young character from the genre’s signature pitfalls. Her evocations of Deming’s plight frequently swerve into freighted cliché: “He couldn’t shake the feeling that something wasn’t right.” “He had lost so much, and he was lost.” Symbols for the lingering pain of Deming’s displacement abound. The twenty-something Daniel spends all his money on a poker game, high on a “savage euphoria” that defies his competitors’ expectations as the losses mount. After a performance with his band, he slips out of the venue. “It felt good,” Ko writes, “being the one making the excuse to get away.” Of course Deming’s pain is profound and real, not least because of his nagging sense that he’s different from other adopted kids—that somewhere along the way, in his conscious memory, he went from being wanted and cared for to utterly cast aside. Yet lacking the context for his own story that readers eventually glean from the portions of the book told from Polly’s perspective, Deming becomes more tragic vehicle than nuanced portrait.

You might expect the same would be true of Polly, Deming’s ambitious and fiercely loving mother—and the one character in the book who sees the inhumane bowels of the American immigration system firsthand. Instead, it’s Polly who steals the show in this book. Though she’s rarely in control, Polly isn’t helpless either as she navigates an existence that confronts her with impossible choices. And she certainly doesn’t fit the profile of the idealized, nurturing mother, the selfless immigrant woman who doesn’t mind doing backbreaking work if it means a better life for her son. Polly is driven by dreams and demands of her own.

Deming, Ko makes clear, was never part of Polly’s plan. Her emigration from China was an attempt to flee marriage to his father, and she’d hoped to lose her baby—by miscarriage or abortion—along the way. His birth did not put her ambivalence about motherhood to rest. In one scene, when Deming is a baby, Polly reaches a breaking point. “Fast now,” she recalls,

before I could change my mind, looking around to make sure no one could see me, I set the bag on the pavement under the bench and lowered you inside. The bag was taller than you, its sides a stiff, insulated plastic. When I got up I was lighter, relieved.

I ran.

She returns, of course, and finds Deming still inside the bag. She sends him to live with her father in China, then saves money to bring him back to New York when he’s old enough to go to school. She cares deeply about her son. But Polly’s simultaneous guilt and relief, skillfully portrayed by Ko, reveal her life as one in which choice and necessity tend to blur. In a very real way, Polly is a leaver. She runs. That quality enabled a village girl to “[make] it all the way to New York”; it enables her, later in the book when she has returned to China, to not go back and find a grown-up Deming when she has the chance.

In the character of Polly, Ko has pushed back against James Baldwin’s famous pronouncement that “the failure of the protest novel lies in its rejection of life, the human being, the denial of his beauty, dread, power.” This is not a novel about a woman who did everything right, only to be punished at the hands of a cruel system. That system is cruel, and its victim is imperfect.

Ko’s compelling book is about a woman who has done lots of things wrong, and lots of things right, and has mostly lived as best she could.