Fourteen-year-old Heather is anticipating the celebration of Halloween. She and her dad decorate their house in the Austin suburbs with streamers and plastic spiders and host a successful Halloween party. Heather dresses up as Wednesday Adams and goes around the neighborhood with her friends, scaring younger kids. 

At the end of the night, Heather whispers “goodnight” to a picture of her mom that she keeps under her pillow. A year ago, her mother vanished without a trace. The next year, her father tells her that they will be going to England to look after a farm left to them by a recently deceased aunt named Rose. Her father is going to sort paperwork and sell the antiques his aunt collected. Heather wants to stay for their Halloween party, but her father tells her the flight can’t be moved. 

Soon after, they land in Heathrow and embark on a long drive in the rain to the farm. On the way, they pass The White Mare, the village pub. The house looks more like a storage building than a home and Heather shivers as she looks at the decaying structure. Suddenly, she gasps as she sees a red horse named Callisto on the other side of the fence. She reaches out to pet the horse and the horse’s owner Chester appears. They introduce one another and Chester tells them how he takes care of Rose’s animals. 

Heather and Callisto become inseparable and she learns how to ride the horse. She explores the town on horseback while her father sorts out the house. One day, Heather stops at a shop to get sugar cubes for Callisto. She asks the cashier for candy, but only gets a look of confusion in response. Then, three teenagers named Harry, Chloe and Ian enter the shop and they tell Heather they’re upset that she and her dad took over the farm and took Callisto, too. 

When she tries to befriend the teenagers and ask what they’re going as for Halloween, they laugh at her. On her way out of the shop, they issue an ominous warning about the farm but say nothing more. When her dad suggests throwing a Halloween party, Heather hesitates and they decide not to host one this year. As their move-back date approaches, Heather’s dad reveals that they’ve made a decent amount of money off the antiques and can afford to get a bigger backyard and bring Callisto home with them. 

On the day of Halloween, Heather rides Callisto in the woods behind the farm and thinks about the villagers’ warnings. Later, during dinner, they hear a pounding on the door. When they open the door, they find a group of people dressed as black-robed demons standing outside. The crowd shouts the name of the nearby pub, “White Horse,” at Heather and her dad, making her dad uneasy. When he tries to shut the door on them, one of the robed figures grabs a hold of the door, and a white figure wearing a horse’s skull walks towards them — the White Mare. The person does a dance and Heather’s dad calls 999, but the operator merely tells him it's a tradition and hangs up. After about an hour, the people leave their house and they find the place relatively unchanged despite the invasion of privacy. 

That night, she dreams that the dancing group jabs her with spikes and laughs at her American accent. In the morning, she senses that something is wrong and finds a note slipped under the door that reads, “THE WHITE MARE TAKES WHEN YOU DON’T GIVE.” When she opens the door, she finds Callisto’s head on a stake. Later, the police come and tell the them no crime was committed, as this is a local custom. The pair buries the horse’s skull. 

In the woods, Chloe and Ian talk to each other about killing the horse and say that Heather deserves the loss. As they turn to leave, they hear approaching hoofbeats. They see Callisto’s skull, attached to an invisible body, staring straight at them. The skull’s jaw closes tight around Chloe’s wrist and invisible hoofs punch her in the stomach. Ian watches on in horror as he’s covered in her blood. After she’s dead, the horse kills him, too. As Heather walks to the car on her way out, she sees the disturbed soil and clicks her tongue three times.