Martin Lowery is a scientist sent to a government-controlled outpost while an unspecified pandemic has ravaged the country. The outpost is located in an unusually fertile forested area outside Bristol to help in the studies and experiments of his former colleague and ex-lover Olivia Wendle regarding using mycorrhiza to increase crop efficiency. After passing a physical examination and meeting his park guide Alma, he learns of the local legend of Parnag Fegg, a woodland spirit. The following morning, Martin and Alma begin their two-day hike toward Olivia's site.Alma informs Martin that Olivia has not been heard from in months. The pair come across an abandoned tent, with the remaining items indicating it belonged to a family. The next night, Martin discovers a rash on his arm before he and Alma are assaulted by unknown assailants who also raid their camp, destroy their equipment, steal their shoes, and loot some of their supplies. Without his shoes, Martin badly cuts his foot.They are approached by Zach, a man living in the woods who offers to lend the pair some shoes. Taking them to his own campsite, Zach disinfects and stitches Martin's wound and gives them both food and a drink. Martin and Alma begin to lose consciousness and realize they have been sedated. While they are unconscious, Zach takes ritualistic photos of them in strange clothes and positions and stitches a strange symbol and animal gut into Martin's arm.Upon waking up Zach describes the symbol as a mark to be seen by a presence in the woods, claimed by him to have once been an ancient sorcerer who placed his essence in a standing stone somewhere in the forest. Martin's foot becomes infected from his injury and Zach sloppily amputates some of his toes with a hatchet.Alma acquires a discarded blade to free herself and Martin before attacking Zach. Martin and Alma flee, but become separated. Zach pursues Martin, who stumbles across a pair of bodies, presumed to be the owners of the abandoned camp. After encountering the standing stone, Martin is found and rescued by Olivia. Alma independently makes her way to the camp, and Zach is driven away by the amplified sound and lights set up around Olivia's site.The next morning, Olivia cauterizes Martin's foot as she explains her project. She reveals that Zach is her ex-husband, and that the pair were following the directions of a centuries-old book on local legends. He had been helping her to try and communicate with the presence inside the stone, using the lights and noises from an electronic soundboard, but disapproved of her approach and left. Unable to trust Olivia and fearing that Zach is still after them, Alma tries to convince Martin that they need to leave, but he will not be drawn on the nature of his relationship with Olivia and she refuses to abandon her research.The next morning, the site is surrounded by a thick mist containing fungal spores, trapping them. They send Alma through the mist in a hazmat suit, but the spores make it through her protective clothing and subject her to horrifying visions. That night, the mist continues to close in on the site as Zach arrives, telling them to communicate with the standing stone using the sounds and lights as well as to consume a "sacrament," a mixture of ground mushrooms alluded to in the book.Martin agrees to drink the sacrament and waits for it to take effect. Zach ambushes Alma and prepares to sacrifice Martin. Alma makes her way back to camp, where she finds Olivia in her tent surrounded by staged photographs of her and Zach's victims. Olivia attacks Alma, but she fends her off. Zach hears their struggle and rushes back to camp, where Alma kills him. She then pursues Olivia back to the standing stone as the mist overtakes the camp, and they are all subjected to more visions from the spores.By morning, Alma and Olivia suddenly find themselves far from the stone, broken out from their trances. Olivia, overcome with awe, collapses on the ground. Alma then approaches a waking Martin by the stone and, speaking with a distorted voice, she offers to guide him out of the woods. (thanks to Wikipedia)