Whilst drilling for oil on a field, a worker stumbles away from his equipment and collapses on the ground, blood pouring from his ears.

A wealthy family — politician Gwyn, his wife Glenda, and their two adult sons, Guto and Gweirydd — host a dinner party at Glenda’s inherited estate in the Welsh mountains. Their guests are to be Mair, a neighbouring farmer who grew up with Glenda, and Euros, a businessman who has been drilling for minerals on the family’s land.

To help cater the party, Glenda has hired a girl named Cadi from the nearby village. Cadi arrives on foot at their remote estate, her hair is wet, and she rarely speaks. Cadi observes the family in silence as she helps Glenda prepare. One son, Guto, has a drug addiction. Early in the film, he accidentally drops an axe and cuts his foot. The other son, Gweirydd, is training for a triathlon and has an obsessive diet and regimen.

Elements of the family’s lifestyle appear to upset Cadi. She becomes upset when Gwyn shoots a pair of rabbits and arrives with the dead rabbits to prepare them for supper. Cadi’s emotional distress has physical consequences, for example a muddy stain appears on a clean tablecloth when she hears the gunshots. Gwyn experiences sudden pain and noise whenever he confronts Cadi. A bottle of wine explodes in Euros’s hand when he arrives and accuses her of slacking off on the job. Afterwards, Cadi picks up a piece of broken glass and appears to insert it into her vagina.

When Guto asks Cadi if she has any drugs to help him get through the party, she takes him into the forest and helps him pick psychoactive mushrooms. He eats some of them before coming to dinner.

Mair arrives, signalling the start of the party. She explains that her husband, Iori, has been delayed by an emergency: an unknown person drove into a nearby lake, and he is helping to retrieve the car.

The atmosphere at the party is awkward. Mair is polite but put off by Glenda’s behaviour. Gweirydd casts doubt on his father’s recent health scare, embarrassing him in front of the guests. Guto expresses disgust at Gweirydd’s raw-meat diet and his parents order him to leave the table. He goes back to his room, mixes and injects the remaining mushrooms.

Cadi, no longer needed, walks back into the forest and lays down in the grass, smiling as vines and flowers come alive and encircle her body, suggesting that her connection with nature is a magical one.

The party’s true purpose is revealed. Gwyn, Glenda and Euros all hope to convince Mair to allow exploratory drilling and mining on her land. The most promising spot is “the Rise,” but Mair points out this is the reputed resting place of a goddess in local folklore. Children are warned to avoid it; even adults refuse to disturb it for fear of waking her. Mair is horrified by the suggestion of mining the Rise and turns them down. Glenda accuses her of believing in superstition.

Meanwhile, Guto’s entire leg has become necrotic and infested with maggots. Cadi seduces Gweirydd and leads him outside to the Rise, where Guto lies helplessly on the ground. Delirious with pain and addled by the drugs, Guto shares the real reason Gweirydd no longer works at the hospital: he raped a number of comatose female patients. Cadi pressures Gweirydd into cutting off his brother’s leg with an axe. Cadi and Gweirydd then have sex against a tree, and Gweirydd screams in pain due to the sharp piece of glass still hidden in Cadi’s vagina.

Back at the house, Mair receives a call from her husband and learns the car found in the pond was Cadi’s. She and Glenda realize the woman who has been in Glenda’s house all day is not the real Cadi — it is the goddess of the Rise possessing Cadi’s drowned corpse. Mair flees, leaving Glenda to her fate.

The goddess returns to the house and kills Gwyn by inserting a skewer into his ear.

A badly injured Gweriydd arrives back at the house, carrying the now dead Guto on his shoulders, and soon collapses and dies from blood loss. Soon after, in an apparent trance, Glenda butchers Guto’s body and serves the maggot-infested leg to the similarly entranced Euros, who is eating leftovers from the feast like a pig. Glenda puts the barrel of a shotgun in his mouth and asks him: “After you’ve taken everything, what will be left?” Glenda pulls the trigger and then slits her own throat.

The goddess returns home to the Rise and burns all the bodies.