In this alternate reality setting, the fighting between the North and South during the American Civil War is halted when a new combatant inexplicably rises from the battlefield at Gettysburg: an undead zombie army. The undead who rise to prey on and infect the living are referred to as "shamblers."

The book follows Jane McKeene, a Black teenager who is training as an "Attendant" at Miss Preston's School of Combat for Negro Girls in Baltimore County. Attendants are combat-trained chaperones, employed to accompany wealthy white women for protection against shamblers. Per the new laws implemented by the Survivalist party, which is currently in power, Black teenagers are forcibly conscripted into various types of combat schools, where they are trained to fight shamblers. 

Woven throughout the narrative as epigraphs are excerpts from correspondence between Jane and her mother. As the story unfolds, these letters, along with Jane’s own recollections, reveal snippets of information about her life before Miss Preston’s. Jane explains that she is the secret but beloved daughter of a wealthy white woman, “Momma,” and that she grew up at Rose Hill, her mother's plantation. Her mother tried to keep her out of combat school for as long as possible, but when the government finally came for Jane, Aunt Aggie—a member of the staff and a mother figure to Jane—gave her a good luck charm: a penny on a string, which Jane wears every day as a necklace. 

Jane insists that the penny has magic to it and grows cold when she's in true danger. This necklace proves useful not only as she undertakes the dangerous work of fighting shamblers, but also as she navigates her precarious position as a Black woman in a deeply racist and stratified society. Jane is an exemplary fighter, taking incredibly well to her combat training, but she bristles against etiquette training and the expectation that she behave in a certain way. This often gets her into trouble at school, where she experiences particular tension with her prim, proper classmate Katherine Devereaux.

When Jackson Keats, Jane's former romantic interest, asks her for help tracking down his missing sister, Jane and Katherine both become embroiled in the mystery. Soon, they're caught snooping around the home of the mayor, whom they believe to be involved. As punishment for their transgression, the three of them are sent west to a new Survivalist frontier settlement known as "Summerland."

Shortly after arrival, Jackson is imprisoned after an ill-advised escape attempt. Before they're sent off on patrols, Jane takes a chance on a strategic ploy: she convinces the sheriff who controls Summerland that the light-skinned Katherine is actually white. Under the watchful eye of the racist Pastor Snyder, Katherine is sent to live with the other whites in the nice part of Summerland. Meanwhile, Jane is sent to join the patrols, where she’s tasked with patrolling the top of the border walls and stopping any shamblers that try to climb over. She soon realizes that despite being the settlement’s best defense against shambler attacks, the Black patrols are treated as expendable. They’re given inadequate weaponry and chronically underfed to save food for the white, wealthy residents of Summerland.

As the girls acclimate to Summerland, it becomes clear that something is amiss. They meet an ally in the form of a tinkerer named Mr. Gideon, and he shows them kindness; however, the rest of the town seems even worse than they anticipated. This purported Survivalist utopia, replete with big political promises, plumbing, and electric lights, proves to in reality be a dangerous wasteland built on the continued exploitation of Black workers.

Unwilling to conform to the rules of the town, Jane finds herself in frequent trouble with Sheriff Snyder. Their feud escalates when Jane is publicly whipped in the town square for sneaking out after curfew. He soon needs her help, however, when the town’s protective wall is breached by a horde of shamblers. Summerland’s makeshift army is small—only thirty people or so—against more than a hundred shamblers, and nobody seems to know what to do. Thinking fast, Jane begins barking orders and leads the townspeople to a miraculous victory. 

After the battle, Jane senses Sheriff Snyder's ire and is surprised when he doesn't immediately punish her. The following day, Mr. Gideon reveals a troubling pattern he's discovered: the shamblers are forming larger groups and coming from further and further away, suggesting that they're transmitting a signal of some sort. Mr. Gideon reveals that several eastern cities, including Baltimore, have fallen to giant shambler hordes and predicts that these attacking hordes will only continue to grow. 

Aware that Summerland won’t survive the next shambler attack, Jane and Katherine make plans to warn the others to evacuate. Before they can, however, they are confronted with a more imminent threat: Sheriff Snyder has learned of their ruse and is enraged by the knowledge that Katherine—whom he was romantically interested in—is actually a Black woman. The situation comes to a head as a new group of shamblers is spotted approaching Summerland. The sheriff and his men capture Jane and Katherine, intending to kill them, but the girls use their combat training to overpower them. In the ensuing struggle, the sheriff, the pastor, and the sheriff’s henchmen are all killed.

As Katherine and Jane flee the sheriff’s office, the shambler horde arrives and begins roaming the streets of Summerland, eating anybody slow enough to catch. Jane and Katherine meet up with Jackson and several of their allies from town, and they all flee Summerland in a wagon. When they're far enough away, Jackson gives Jane a letter from Momma, explaining that he got the letter from a Miss Preston's girl in a nearby town.

In the letter, Momma reveals that Rose Hill Plantation has fallen to the Shamblers and that she's fled to Haven, a Survivalist settlement in California. After her experience in Summerland, Jane realizes that humans will never defeat the shamblers so long as the racist and selfish Survivalist party remains in power. Determined to change things, Jane decides to set off for California.