Set in England during a record-breaking heat wave in the summer of 1989, the novel follows Mina Ellis, a recently graduated child psychologist in her early twenties, struggling with unresolved grief over her younger brother Eddie, who died of pneumonia at fourteen nearly six years earlier. Mina is engaged to Oscar, an older, cautious space researcher, but she feels trapped in their relationship and secretly begins taking birth control pills. When she discovers a blurred figure resembling Eddie in the background of a holiday photograph, she becomes convinced her dead brother is fulfilling a deathbed promise: If anything existed beyond death, he would come back and tell her.
Oscar dismisses her belief and urges her to return to a bereavement support group. There, Mina meets Sam Hunter, a grief-stricken journalist whose seven-year-old daughter, Maggie, died of measles. A week later, Sam invites Mina to a café and describes a case he is investigating: A teenage girl named Alice Webber in Banathel, a small, impoverished Cornish village, has been experiencing disturbing symptoms since the previous Christmas. Alice claims a witch with a black tongue and an upside-down face watches her through cracks in her chimney breast. Sam played back a tape recording from Alice's bedroom and captured a disembodied whisper saying "Good riddance!" when no one else was present. He asks Mina to assess Alice as a child psychologist, and she agrees over Oscar's objections.
Sam drives Mina to Banathel, a close-knit village steeped in folk superstition. Hagstones, pebbles with natural holes believed to ward off witches, hang in nearly every doorway. Outside the Webbers' house, a crowd has gathered, convinced Alice can communicate with the dead, pressing photographs and personal items through the letter box for Alice to use in contacting their deceased loved ones. Lisa, Alice's exhausted mother, welcomes Mina inside. Mina meets Alice and finds her surprisingly ordinary: a tanned, blond teenager listening to music and reading horoscopes. Alice gradually reveals the origin of her troubles. The previous winter, her best friend Vicky lured her to an abandoned house, where schoolmates pressured her into reaching into a chimney to retrieve a bottle. When Alice dropped it and it shattered, Vicky told her she had broken a "witch's bottle," a folk-magic artifact used to trap malevolent spirits, and the two stopped speaking. Mina suspects mass psychogenic illness, but Alice insists the haunting is real.
Mina and Sam visit the abandoned house and find apotropaic marks, protective symbols against witches, burned into the rafters. Sam discovers a child's shoe in the chimney identical to ones his dead daughter wore. A scratching sound begins inside the chimney, and Mina, certain the shoe is a trap, drags Sam out. Alice's father, Paul, a stocky slaughterhouse worker, later admits to exaggerating some of Alice's symptoms because the family is drowning in debt and desperate for media attention. Mina discovers a drawer full of overdue bills. Yet her skepticism wavers when Alice demonstrates uncanny knowledge of her personal history, using Eddie's nickname "Meens" and referencing the frozen pond where Eddie once saved Mina's life.
Tensions escalate when Vicky Matherson arrives with teenagers who hurl eggs and taunts. Alice walks outside with a blank expression, and Vicky collapses, her throat swelling shut from a wasp sting. She is hospitalized with anaphylactic shock. Sam conducts a séance with Alice, who enters a trance and speaks about Maggie with details Sam never shared, accusing him of abandoning his daughter. Violent scratching erupts at the kitchen door, and when Sam opens it, deep claw marks have been gouged into the wood. Later, Simon Pascoe, one of the teenagers who harassed Alice, drowns in a local quarry. The town grows fearful, and Lisa tells Mina a Riddance, a traditional ritual to drive out the Devil, may be necessary.
Bert Roscow, the Webbers' elderly neighbor and a former journalist whose wife, Mary, is gravely ill, has been educating Mina about Banathel's history of witch persecution. Mina visits their house with Alice, who fondly recalls Bert and Mary caring for her as a child, making cocktails called "Bertinis." When Mina sneaks upstairs, Mary struggles to speak, managing only "Bill—" before Bert ushers Mina out. Fern, the owner of the local video shop, reveals that she too was taken in by Bert and Mary as a troubled teenager, as was Lisa. A pattern emerges: Bert has sheltered multiple vulnerable adolescent girls.
One evening, Bert arranges for Sam and Alice to leave the Webber house. Left alone, Mina enters Bert's basement and discovers a tailor's dummy dressed in a faded blue Riddance dress weighted with hagstone necklaces, and a box containing a witch pricker, a long needle inscribed with the Latin phrase DAEMONIA EICERE ("cast out devils"), and blackened metal pincers. Alice screams from upstairs; Mina finds Mary dead, showing signs of asphyxiation. A Billie Holiday record plays on Bert's turntable, and Mina finds an envelope taped inside the record sleeve. She realizes Mary's halting word was not "Billy," the Webber boy's name, but "Billie," directing Mina to hidden evidence. Inside are Polaroid photographs of unconscious adolescent girls in states of undress, drugged with spiked Bertini cocktails. Among them, Mina identifies teenage Lisa, a young Fern, and Alice.
Bert discovers the photographs are missing and locks Mina inside Billy's room, threatening to expose a secret about her past. Mina climbs out the window and flees to Fern's shop, but Fern reacts with furious denial and shoves Mina out. Bert tells the Webbers and Sam that Mina cannot be trusted. Sam, shaken by footage of Alice apparently transforming during a trance, sides with Bert. Mina breaks back into the basement and puts on the Riddance dress. Bert traps her, stabs her with the witch pricker, and demands the photographs.
Bert forces Mina, weighed down by the dress and necklaces, out to the village green, where townspeople have gathered around towering bonfires for the Riddance ceremony. He drags her into the village pond, forces her jaw open with the pincers, and begins tearing at her tongue, enacting the historical practice of splitting a witch's tongue. Alice, who has seen the Polaroids and learned the truth, leaps through a bonfire and drives the witch pricker into Bert's neck. He staggers into the fire. Mina, weighed down by the waterlogged dress, sinks beneath the pond and is clinically dead when Sam pulls her out.
Mina wakes in the hospital six days later. Fern explains that after throwing Mina out, she retrieved the Polaroids and showed them to Lisa. Together they told Alice, who discovered the witch pricker's retractable mechanism, proving Bert used it as a trick, and resolved to stop him. Vicky died in her coma. The Webber house caught fire from the chimney, and the family fled north. Alice sent Mina a postcard with Eddie's photograph, writing "He 4gives U!" on the back. Oscar ended their engagement.
The novel's central secret emerges through Mina's memories. After Eddie rescued her from the frozen pond, the cold worsened his already fragile health, caused by a poorly formed immune system, into fatal pneumonia. In his final hours, he whispered for Mina to tell him the story of the ice "so I'm not afraid while you do it" (290). Mina lowered a pillow over his face, ending his suffering as an act of mercy. This is the act Alice's witch-voice referenced. In the final scene, Mina walks to Bert's private room in the Trelawney Wing, where he lies badly burned but expected to survive, slides the pillow from beneath his head, and whispers, "Goodbye, Bert. Good Riddance" (292).