In a contemporary United States where witchcraft is widely believed to be real, women face severe legal restrictions. Unmarried women over 28 must register with the Bureau of Witchcraft and submit to quarterly monitoring. State-sanctioned witch burnings persist, and marriage offers women protection from state scrutiny.
Josephine (Jo) Thomas is a biracial woman in her late twenties who works at the Museum of Cursed Art of Southern Michigan. Her Black mother, Tiana, disappeared when Jo was 14, and Jo's white father raised her alone. After years of searching, the family agrees to declare Tiana officially dead. Jo visits a storage unit containing Tiana's belongings and finds two handmade dolls resembling her as a child and a yellowed clipping about an island in Lake Superior that appears once every seven years.
Jo has been seeing a man she calls "Party City" in her phone, whose real name is Preston. After an emotionally raw night together, when Jo reveals her family has declared her mother dead, he suggests they build a real relationship.
Jo's father calls with unexpected news: Tiana's updated will leaves Jo approximately $133,740 in invested lottery earnings, but Jo must travel to a small island in Lake Superior on one of four specific dates, the nearest being in four days. She must gather purple apples from a special tree and bring one of the dolls. Meanwhile, her boss reminds her to complete mandatory registration as an unmarried woman approaching 28.
Jo and her childhood best friend and roommate, Angie, drive north through Michigan. They pass through hostile towns with signs reading "WE BURN WITCHES HERE" and are questioned by police suspicious of two women traveling without men. At a party near Marquette, Jo encounters a mysterious blue light by the lakeshore that draws her toward it before she pulls back.
A boat captain who knows about Tiana's arrangements takes Jo alone across Lake Superior. Gold droplets fall from darkening skies. An island appears through the storm, with white sand, trees bearing vivid purple fruit, and a statue of a woman with moose antlers. On the shore, smiling and waving, is Tiana.
Jo embraces her mother, overwhelmed by gratitude and fury. Tiana explains the island is a place where magic enters the world safely, sustained by nature and community. She reveals she came seeking treasure and freedom, unable to bear living under laws that coerced women into dependence, and urges Jo to stay.
Tiana shows Jo women who fly, transform into animals, heal plants, and create art through spells. She reveals that everyone is capable of magic; the lie that only women possess it serves as a tool of control. Jo meets Linden (Lin), a woman born on the island who teaches her spell work. At a nighttime ceremony, Linden applies mashed purple fruit to Jo's feet, and Jo flies for the first time. While airborne, she sees a bright pink door in the sky that opens to reveal gray nothingness and a terrible scream, and she wills herself back toward the shore.
Over the following weeks, Jo studies spells in the island's library, sews protective dolls, and grows closer to Linden. She discovers a drawing of the pink door, annotated with warnings by many hands, and finds a spell called Extinguish: It depicts a woman being burned at the stake who meditates on rain, glaciers, and waves and flies free from the flames.
Jo's relationship with Tiana remains strained. Tiana dismisses Jo's life back home and refuses to apologize for leaving. After a bitter argument in which Jo pours out years of pain about being investigated by the Bureau as a teenager, bullied in high school, and devastated by her mother's absence, Jo decides to leave. At the dock, Linden kisses Jo and wraps a protective bracelet of dune grass and seaweed around her wrist. Tiana does not come to see her off.
Jo returns to the mainland, where only a week has passed despite six weeks on the island. She is immediately taken into custody by the Bureau of Witchcraft. Two agents, both calling themselves Bill, subject her to days of interrogation, asking whether she has eaten a baby and whether the Devil is her true father. She endures physical tests, including submerging her hands in holy water, being pricked with silver needles, and repeated dunking in a lake. She passes every test.
Preston, contacted by Angie after Jo's father refuses to come, signs custody papers for Jo. They move into a small house under severe restrictions: biweekly check-ins with a monitor, no gatherings of more than two women without male supervision, and the threat of imprisonment for any infraction. Registering as a witch would mean forced sterilization, being barred from having money in her own name, and lifelong monitoring. Jo returns to the museum, where she can now sense magic in certain artworks but keeps her reactions hidden.
Jo and Preston grow genuinely close, and he tells her he loves her. Yet Jo lies awake knowing her job, home, and freedom all depend on him, unable to untangle love from dependence. Her father sends a letter calling his marriage to Tiana his greatest mistake and saying he will welcome Jo back only when she recommits to being good. Angie announces she is getting married through a service that pairs gay women with gay or asexual men.
One winter night, Linden visits from the island. When she touches Jo, suppressed magic surges through Jo and she involuntarily transforms into green glass, her bones and veins visible through translucent skin. Linden reverses the transformation, but a neighbor has filmed it and called the Bureau. Jo is arrested, and neighbors pile on accusations.
The Bureau tells Jo's family a trial will happen in months but privately informs Jo she will be burned at the end of the week. Angie visits, and Jo hides the truth, joking with her friend one last time. Preston visits in a suit. Jo confesses she is a witch and tells him she has feelings for Linden. He admits he has wondered whether Jo's love arose only from needing him. Jo acknowledges she cannot untangle that question as long as her safety depends on him.
On a cold night, officials gather on a private beach. Jo is sedated, dressed in a black robe and pointed hat, and tied to a birch pillar over kerosene-soaked logs. As the fire is lit, the pink door appears before her, silent this time. She hears women's voices singing in harmony. Channeling the Extinguish spell, Jo meditates on glaciers, rain, and waves. Steam rises between her and the flames. Her bonds burn away, and she soars into the air as fog spreads from the lake, echoing her ancestor who centuries earlier flew from her own burning.
Jo flies to Angie's childhood bedroom to say goodbye and invite her to the island. Angie reveals she already knew Tiana was alive: She pulls a box from under her bed containing two dolls resembling them both, sent by Tiana as invitations.
The novel closes on a June day. Jo and other witches dig soil and lay stones, herbs, feathers, and cement to build the magical path she envisioned with Linden, one invisible to those who wish harm. Jo addresses an unnamed person fleeing their former life, describing the path glowing silver and white and pink and green, promising food, blankets, and a place to start again.