In January of 1936, 19-year-old Cussy Mary Carter’s father is trying very hard to find a suitable husband for his daughter. Cussy’s mother died of influenza and Pa—a Kentucky coal miner—is sick himself with black lung. Cussy doesn’t want to get married; she has a job with the WPA’s Pack Horse Library project, delivering books to people in remote communities and homesteads in the hinterlands around Troublesome Creek. She’s also afraid of the kind of man who would want to marry one of the “Kentucky Blue People.” Cussy and several generations of her ancestors have a rare genetic condition that turns their skin blue, and although they are otherwise healthy and normal, they are treated with fear and bigotry. The only reason Cussy has any suitors at all is because Pa has offered 10 acres of land for her dowry. He finally gets a taker, an old man named Charlie Frazier. Charlie mercilessly beats Cussy on their wedding night, then fortunately dies of a heart attack. Cussy, now in possession of Charlie’s abused mule, Junia, returns to her work as a Pack Horse Librarian.
Cussy brings books to far-flung homesteaders—including Mr. Moffit and his wife, Angeline; Martha Hanna and “Devil John” Smith; Miss Loretta Adams; and Oren Taft—and to the mountain school run by Winnie Parker. And although she faces some bigotry from her patrons—Mr. Moffit doesn’t like to look at her blue face—overall, Cussy’s patrons like her. Some, especially Angeline, Winnie, and Miss Loretta, consider her a friend and don’t care about the color of her skin. And when she meets her newest patron, Jackson Lovett, she quickly earns his respect and regard. The same isn’t true of her supervisors, Eula Foster and Harriet Hardin, who treat Cussy and the only Black librarian, Queenie Johnson, as unintelligent, disease-ridden pariahs because they are “colored.”
After Cussy returns to her library route, her dead husband’s cousin, Pastor Vester Frazier, begins to stalk her. Vester is well-liked by many in the community, but he has a long history of harassing, harming, and killing people who are different (twins, a little person, and the Blues) because he believes they are the result of demonic forces. Vester claims that Cussy is a “blue witch,” and that she’s responsible for Charlie’s death. One day, he attacks Cussy in the woods and tries to rape her, but Junia manages to chase him off just in time. Soon after, he tries to sneak onto the Carter property while Pa is at work, but Junia breaks free from her stall and tramples him to death.
The only people who know about what happened to Vester are Cussy, Pa, and Doc, whom Cussy and Pa called to treat his injuries before he died. Doc uses this opportunity to blackmail Pa and Cussy into participating in medical research, since he has long been curious about the family’s blue skin. In exchange for his silence, he wants to take Cussy into Lexington once a month to the hospital, where he performs traumatizing and degrading tests on her. Once he realizes the root cause of her skin coloring, Doc offers her a drug that’s capable of turning her skin white, at least temporarily. Cussy jumps at the opportunity to become “normal,” and accepts the horrific side effects of the medication for the chance to be accepted in her community.
But when Cussy tries to attend Troublesome Creek’s Fourth of July celebration as a “white” woman for the first time in her life, the townsfolk still shun her. Following this embarrassing fiasco, she resolves to live as herself, even if that means she stays Blue and continues to face discrimination and difficulty.
Cussy’s friend and patron, 16-year-old Angeline, is pregnant with her first child. Angeline and her husband Mr. Moffit live alone, as neither of them have any kin left in the area. She’s excited about having a baby, and she’s convinced it will be a girl. She plans to name it Honey. But when Honey is born, the baby is Blue. Mr. Moffit, it seems, is the illegitimate child of one of Cussy’s great-uncles; somehow, Angeline must also carry the recessive gene. Horrified by his child, Mr. Moffit hangs himself in the front yard on the day that Honey is born. Cussy finds him there while on her library route, and she rushes inside to find Angeline dying of a hemorrhage after giving birth. As she lies dying, she begs Cussy to raise Honey for her.
At first, Pa worries that being a single mother will just make Cussy more vulnerable, but he eventually warms to the baby and understands why Cussy—who knows so well the pain and trauma of growing up Blue—wants to care for her. Shortly after Honey’s birth, he tells Cussy to expect a new suitor, since Honey needs a father. But Cussy would never have expected that suitor to be Jackson Lovett, who has been in love with her since he first laid eyes on her in the spring. They share a love of books and an understanding of pain and trauma. The night Cussy accepts his proposal is the night that Pa dies in a mine collapse; he dies without knowing that Cussy will be taken care of in the way he wanted her to be.
In October, Jackson and Cussy get married in the Troublesome Creek courthouse. They leave the ceremony and are greeted by a crowd of Cussy’s patrons, who have come to wish them well. But then, Jackson is arrested by Sheriff Davies Kimbo for breaking the anti-miscegenation laws that prohibit white people from marrying “colored” people. Although Doc attests that Cussy is a white person with a medical condition, Sheriff ignores him and gives Jackson a vicious beating for his failure to comply. A growing crowd looks on, some supporting Cussy and Jackson, others supporting Harriet and the Sheriff in their bigoted attack.
Four years later, Cussy Mary writes a letter to Queenie Johnson, who has moved to Philadelphia and is going to school to become a proper librarian. Jackson was eventually released from jail and recovered from his injuries. He has been living in Tennessee and visiting Cussy secretly while he sold off his land and they prepared to move to Ohio, where they can live together openly. Honey loves her parents and is learning to read. She is eager to become a book woman herself one day.