Set in a world where a feminine nature deity called the Old One endows certain women with supernatural gifts to counterbalance mankind's destruction of the earth, the novel traces six generations of the Duncan family, a lineage of witches bound to Wild Hill, an estate on an island off New York State. The story is framed by Bessie, a 17th-century Scottish midwife whose ghost has guarded Wild Hill for 400 years, watching over the Duncan bloodline until it produces "the most powerful of our kind," a prophesied trio known as The Three (ix).
Brigid Laguerre, a filmmaker known as the Queen of the Dark, possesses the gift of seeing how people will die. She has refused to kill on the Old One's behalf for 30 years, ever since her mother, Flora Duncan, died by suicide. When Brigid learns that Calum Geddes, Flora's last boyfriend and a billionaire media mogul, has died, ravens appear as omens. After an arsonist destroys her California home, Brigid kills him. A raven delivers a cast-iron skeleton key, and Brigid flies to New York, answering the call to return to Wild Hill.
In Texas, Brigid's estranged younger sister Phoebe Duncan, a healer who communicates with animals, runs a ranch with her partner Ed Fox. The Old One sends a feral hog named Petunia bearing a message: Phoebe must go see Brigid. Phoebe refuses, but after a tornado destroys the ranch, she accepts that the Old One will not take no for an answer.
Flashbacks trace the Duncan lineage. In 1983, Flora teaches her daughters they descend from Scottish witches. Brigid experiences her first death vision at age six when she foresees a neighbor boy's drowning. During summers on Wild Hill, Aunt Ivy, the family's centenarian matriarch, trains the girls to understand their gifts. Phoebe receives a silver key from a raven, and Bessie tells her it will not work until Phoebe's daughter arrives, revealing that Phoebe's future daughter is The Third. Flora and Phoebe keep this secret from Brigid, fearing her jealousy.
Bessie's backstory reveals Wild Hill's origins. She crossed the Atlantic with colonists, settled on the hill, and was accused of witchcraft and hanged on Culling Pointe, a nearby spit of land. Her spirit returned to Wild Hill, and the colony that killed her descended into cannibalism. For centuries Bessie waited until Sadie Duncan arrived from Scotland in 1898, the ancestor the Old One had foretold.
Each generation carries forward the family mission to topple tyrants, protect the earth, and avenge the wronged. Sadie kills her abusive husband with a lightning bolt. Her twin daughters, Ivy and Rose, poison their predatory uncle and bury him in the basement. Rose's husband develops a dangerous pesticide and dismisses her prophetic warning of environmental catastrophe; the sisters poison him as well. Rose dies in childbirth delivering Lilith, who chooses chemistry as her weapon and discovers that mushrooms in the root cellar produce an undetectable poison. With her husband, a refugee chemistry professor, Lilith spends decades assassinating nearly 500 escaped Nazis and fascists.
The family fractures when Calum enters Flora's life. After Aunt Ivy's death, a burst pipe at Wild Hill leads a plumber to discover skeletons in the basement, and Flora sends the girls to New York for a Barnard College summer program. Calum finds two vials of Lilith's mushroom poison in the attic and uses one to murder his former business partner, reclaiming control of AMN, the media empire they cofounded. Bessie presents Flora with a choice: resume her life with Calum, or sacrifice herself so her descendants can fulfill the prophecy. Flora drinks the remaining poison.
Grief curdles into mutual blame. Phoebe reveals the secret about The Third, and Brigid, feeling betrayed, leaves for California with her father. Phoebe rejects Wild Hill. Their estrangement deepens when Brigid's stepmother intercepts a letter Phoebe sends begging for reconciliation, cementing the rift for 30 years.
Phoebe settles in Texas and raises her daughter Sibyl Duncan-Fox in deliberate ignorance of the family's history. At 24, ravens lead Sibyl from Brooklyn to Wild Hill, where Bessie names her The Third and instructs her to lie on Sadie's grave. The ancestors share their stories through visions. Flora's ghost reveals her death was a willing sacrifice.
The sisters reunite on Wild Hill for the first time in 30 years. Their hostile exchange ends when Sibyl announces herself as The Third. Flora's ghost appears at a bonfire to reveal visions of Calum's theft of the poison and Bessie's proposition. A boat crashes offshore, and Liam Geddes, Calum's son, swims ashore after scattering his father's ashes. Brigid feels an immediate attraction, and the two begin an affair.
Phoebe and Sibyl enter the mansion using the silver key and discover a vast field of the deadly mushrooms in the root cellar. Through Liam, the Duncans infiltrate the network of powerful men Calum assembled and begin eliminating targets, including a corrupt senator and a cattle mogul destroying the Amazon. Sibyl discovers that Lily, her lifelong imaginary companion, is the ghost of her great-grandmother Lilith, who confirms Sibyl's unique gifts: She can access ancestral knowledge, speak with the dead, and her cooking compels people to eat whatever she makes.
The sisters reconcile after learning their estrangement was built on a lie. When conspiracy theories link Brigid to the deaths, Liam invites The Three to a final party, requesting Sibyl's mushroom dishes. The party is a trap. The guests are exclusively powerful men. Liam presents evidence linking Brigid to the deaths, and a Supreme Court justice presides over a kangaroo court. Brigid takes all blame to protect Phoebe and Sibyl. The men sentence her to drowning, echoing the "swimming test" for accused witches. Brigid, who foresaw her own drowning in her first childhood vision, does not fight.
In the kitchen, Sibyl has laced every dish with the deadly mushrooms. When Phoebe pulls her away, the men raid the kitchen and eat the food. Liam reveals to Phoebe that Bessie sent him decades ago, instructing him to inherit AMN, earn these men's trust, and bring them together for this reckoning. His betrayal of Brigid was part of the plan.
On the boat, Phoebe places her hand on Brigid's heart and gives half her own vitality. Brigid's heart restarts. Inside the house, men collapse as the poison takes effect. Survivors pursue the women, but Sibyl, guided by Sadie's ghost, summons a massive rogue wave that destroys the Geddes mansion and washes every remaining man away.
Brigid is alive but transformed. Phoebe, diminished by her sacrifice, tells Sibyl that Wild Hill is now hers. On the beach, dozens of witches stand waiting, drawn to the Island. Bessie narrates her departure: She walks into the waves and finds Gerald, her husband, at the bottom of the sea. Sibyl reopens her Brooklyn restaurant for two guests, a billionaire and a politician. As they describe their vision for a world ruled by a few powerful men, Sibyl serves them a drink made from a family recipe. A storm seals the doors. Their minds grow fuzzy. Sibyl tells them it is too late. The tide has already turned.