Noemí Taboada receives a summons from her father while at a lavish costume party. She cuts her date short and returns home, where her father hands her a mysterious letter from Catalina, her cousin. Catalina claims that her husband’s family treats her cruelly, that she’s being kept as a prisoner, and that she has been seeing ghosts. Noemí’s father dismisses Catalina’s claims as melodramatic but nevertheless thinks Noemí should go visit her cousin in El Triunfo. If she does, he’ll finally let Noemí study anthropology in graduate school. Noemí agrees and leaves the next morning.
When Noemí arrives in El Triunfo, she finds it to be a ramshackle town. An automobile is waiting for her at the train station, and the English man driving it introduces himself as Francis Doyle, great nephew of Howard Doyle. He takes Noemí to the Doyle estate, which everyone calls High Place. Noemí walks into the old, Victorian style home and meets a stern-looking woman named Florence Doyle, who explains that she runs High Place and that Noemí should make sure to follow the rules of the house. Florence takes Noemí to Catalina’s bedroom. Surprisingly, Catalina doesn’t look that sick—even though she claims she has tuberculosis. Catalina tells Noemí that she was delirious with fever when she wrote the letter, so Noemí shouldn’t worry too much about it.
At dinner that night Noemí meets the ancient-looking Howard Doyle. Howard believes in a pseudoscience known as eugenics, and he quizzes Noemí about it, much to her discomfort. Noemí tries to learn more about her cousin’s illness from Virgil Doyle, Catalina’s husband, but he isn’t very forthcoming. The next day, Francis gives Noemí a tour of the grounds, which ends at the English cemetery. The Doyles own a silver mine—which isn’t in operation anymore—and many miners died from an epidemic decades ago. The English miners are all buried here, behind High Place.
The next time Noemí sees Catalina, she seems more lucid, and she secretly asks Noemí to visit a local healer named Marta Duval, who’s been making her a tincture. Noemí agrees to help, and the next day she convinces Francis to drive her to town. She meets Marta at her home, and the old woman tells her that it’ll take one week to prepare the tincture. But the medicine won’t help, she tells Noemí, because the Doyle family is cursed. Years ago Howard’s daughter, Ruth, went on a shooting spree in High Place and killed many of her family members, including her fiancé and mother. The whole place is evil, Marta believes.
While in town Noemí convinces the local physician, Dr. Camarillo, to come to High Place and examine Catalina. After his examination, he tells Noemí that he agrees with her—he doesn’t think Catalina has tuberculosis, and she should probably see a psychiatrist. Noemí tells this to Virgil, but he’s dismissive of the local doctor. The Doyle family has their own doctor, Dr. Cummins, and he’s good enough to treat Catalina. Besides, Catalina is his wife, and he’ll decide what’s best for her.
Noemí and Francis begin to spend more time together. They go for walks, talk about each other’s lives, and Francis shows Noemi his collection of spore prints. That night Noemí has a terrible nightmare: she sees Ruth walk through the hallway holding a rifle, find Howard’s bedroom, and shoot him. Ruth then shoots and kills herself. Noemí wakes up and realizes that she has been sleep walking—something she hasn’t done since she was a child.
Noemí retrieves the tincture from Marta Duval and brings it to Catalina. But immediately after swallowing some, Catalina begins to seize. When the episode passes, Dr. Cummins tells Noemí that she gave Catalina an opium tincture that could have killed her. To everyone’s frustration, Noemí refuses to share where she got the tincture, since Catalina made her promise to keep it a secret.
Noemí visits Dr. Camarillo again as soon as she is able. He tells her that Marta’s tincture could not have been opium—those plants don’t grow around El Triunfo. Noemí’s not sure what to make of this information, so she waits for an opportunity to speak with Catalina. But Florence will no longer allow them to see each other unsupervised. Catalina slips Noemí a note, which she later reads in her bedroom. The note is a page ripped from Ruth’s diary. Ruth wrote about how Howard had beaten and abused her. She planned to kill him so that she could escape from High Place. Noemí is shocked by the note; contrary to what she’s heard, Ruth doesn’t sound crazy or possessed by evil at all. She wants to learn more about Ruth, so she goes in search of Francis.
Noemí finds Francis in his bedroom and tells him her theory: she doesn’t think that the house is haunted, or that Catalina and Ruth are crazy. Rather, she thinks that there’s something in the walls—some kind of chemical or toxin that makes people sick. Francis stands up abruptly and tells Noemí to stop talking. He speaks to her in Spanish, which none of the other Doyles speak, and tells her that she needs to leave High Place immediately. It isn’t safe. A loud moan interrupts their conversation. Francis tells Noemí that Howard’s condition is worsening, and that he needs to go and help him.
Noemí has a dream that Virgil forces himself on her while she’s in the bathtub. She has nightmares almost every night, and her sleepwalking is getting worse. She feels exhausted during the day, and she’s beginning to hallucinate. Noemí tells Virgil that she’s ready to go back to Mexico City—she needs to get away from here and clear her head. Virgil agrees to drive her to the train station tomorrow morning. That night, the Doyles assemble in Howard’s bedroom so that Noemí can bid him farewell. When she walks in, Noemí is shocked to see Howard lying on his bed naked. Virgil forces Noemí over to Howard, who grabs her head and shoves his tongue into her mouth.
Noemí experiences a series of visions that show her the history of High Place. Howard arrived here more than three hundred years ago, deathly ill. But the indigenous people fed him mushrooms from a nearby cave that healed him. Howard grew stronger, and soon he killed the indigenous people and took the land and mushrooms for himself. Noemí wakes from this vision in her bedroom, with Francis pressing a glass of water to her lips. She’s angry with him, but Francis begs her to listen to him for a moment. He tells her that, a long time ago, Howard found a fungus that extends human life. That fungus now grows all over the house, like a spider’s web, and it can even preserve the thoughts and memories of people who have died. They call this web the gloom, he explains. Howard uses the gloom to live forever—when his body is about to die, he transfers his consciousness to the gloom, and then from there into the body of a willing recipient. The next recipient will be Virgil, Francis says. It’s important to Howard that the bloodline is kept pure, but lately the Doyle’s have had trouble producing viable offspring. That’s why Howard wants Noemí to become part of the family. Noemí protests, but Francis tells her that it’s too late. No one can escape from Howard.
That night, Noemí has another nightmare. A woman gives birth in a cave, then Howard grabs the child, kills it, and eats it as part of a ritual. The woman is swaddled tightly and buried alive. Noemí wakes the next day, glad to see daylight. She runs to the front door, which she finds unlocked, and flees from the house. The farther she goes, though, the more she struggles to breathe. She collapses, and Virgil comes to fetch her and brings her back to her bedroom. He throws her in the bathtub and tells her to take her clothes off, otherwise he’ll hurt her. Once she’s naked Virgil begins to touch her, but Francis opens the bathroom door and interrupts, much to Noemí’s relief.
When Francis and Noemí are alone in the bedroom, he tells her his plan to break Noemí out of High Place. Howard’s body is going to die soon, and they’ll use that moment to escape through a tunnel beneath the house. Noemí makes Francis promise to help Catalina too, and he agrees. They should be able to escape in a few days, but until then Noemí has to play along with the family’s wishes. Howard expects her and Francis to get married, and he’s planning a ceremony for tomorrow night.
Noemí is forced to wear an ancient wedding dress to the ceremony, which takes place in Howard’s bedroom. Howard reads a few verses in Latin, gives Francis and Noemí a mushroom to eat, and then sends them off to separate bedrooms. Noemí waits for Francis in her room, but instead Virgil walks in. He says that the family figured out about their escape plan. He tries to force himself on Noemí, but she pushes him over and knocks him unconscious. She then runs to Catalina’s room and meets with her cousin and Francis. The three begin to leave, but they’re caught in the hallway by Florence, who has a rifle. Florence leads them to Howard’s room, where it’s revealed that Francis—not Virgil—is going to receive Howard’s consciousness. Howard begins the ritual, and Francis meekly complies.
Everyone is distracted by the ceremony, so no one notices Catalina grab a knife and walk over to Howard. She stabs him repeatedly and then tries to escape. Florence attempts to shoot Noemí, but Francis wrestles the rifle from her and shoots his mother. Noemí then takes the rifle from Francis and, like Ruth before her, shoots Howard. She then runs from the room with Catalina and Francis, toward the tunnel that Francis had planned to take them through. The tunnel leads to a cavern beneath the house—the same one that Noemí has seen in her dreams. Noemí pulls back a curtain behind the cavern’s altar and finds the mummified remains of Agnes, Howard’s first wife. Agnes’s body serves as the epicenter of the gloom.
Virgil strolls into the chamber and reveals that he planned for all of this to happen—he wants his father to die so that he can have full control over the gloom and High Place. He’s going to bring the three of them back and become the new patriarch of the family. Francis attacks Virgil, and while the two struggle, Noemí grabs a lantern and throws it at Agnes’s body, igniting it in flames. With their precious fungus burning, Virgil and Francis collapse, and Catalina stabs Virgil before helping Francis and Noemí escape. They leave the cavern and run down the mountain, eventually finding refuge at Dr. Camarillo’s clinic, where they wait for Noemí’s father and the police to arrive.