In the opening scene, a man named Morris (Craig Wroe) is working late in his office. He starts to experience pain in his chest. Morris goes to get some water when the pain gets worse. He then suffers a heart attack and collapses on the floor as he dies. We meet Mr. Lockhart (Dane DeHaan) on a train headed to work. He is very much into his work and seems rude and careless to others. Lockhart goes to a meeting with the higher-ups - Hank Green (David Bishins), Hollis (Lisa Banes), and Wilson (Carl Lumbly).
They review a note sent by the company's CEO, Roland Pembroke (Harry Groener), who was supposed to be away for two weeks at a spa in the Swiss Alps, but has not returned since, and as the note indicates, he appears to have had some sort of breakdown. Green shows Lockhart an illegal document he handed in that is now under investigation by the SEC. Lockhart is ordered to bring Pembroke back unless he wants to face the penalty for his action. In a flashback, we see Lockhart visiting his elderly mother (Rebecca Street) at a retirement home.
She carries a small music box with a ballerina that she says is dreaming, only she doesn't know that she is in a dream. Lockhart's mother dies and is cremated before his departure, and he kept the ballerina. Upon arriving in Switzerland, Lockhart takes a ride from a taxi driver named Enrico (Ivo Nandi). While riding up the hill to the spa, a villager chucks a drink at the car. Enrico explains that the villagers have a bad history with the people that live on the hill. There is a story of a baron who wanted to protect his bloodline by marrying his sister. After learning she was infertile, the baron began performing experiments on the villagers, which led to them revolting and burning the baron's sister alive. Lockhart arrives at the spa, but visiting hours have just ended.
He finds Pembroke in the bath house where he goes for a swim. Lockhart urges Pembroke to go home with him. Pembroke appears fine and not interested in leaving. Lockhart leaves with Enrico and tells him to take him to a hotel to make a phone call since he has no service on the hill. Another flashback shows a young Lockhart in a car with his father. His father drove over a bridge on a rainy day as he hears on the radio that stocks have plummeted and a bunch of jobs are gone. He pulls over and walks over to the edge of the bridge. Lockhart watched as his father jumped after leaving the boy in the car.
On the way to the hotel, a deer runs out from the woods and into the road. Enrico hits the deer, which gets stuck in the windshield and causes the car to swerve off the road and crash into a ditch. Lockhart awakens three days later in a room at the spa. He meets the spa's director, Dr.
Heinreich Volmer (Jason Isaacs), who points out that he suffered a broken leg as a result of the accident. Volmer says he notified Green about the accident. He tells Lockhart to try out the spa's treatments during his stay. He is put to stay in a room where he hears the toilet handle rattling by itself.
Lockhart also takes a drink of water and finds a parasitic creature floating in the glass. Lockhart meets three patients - Frank Hill (Tomas Norstrom), Ron Nair (Ashok Mandanna), and Victoria Watkins (Celia Imrie). Watkins in particular enjoys crossword puzzles. The three of them all seem perfectly pleased with their treatments. Lockhart then meets Hannah (Mia Goth), a young woman who considers herself a special case. She and several other patients, plus Volmer and other staff members, are frequently seen taking vitamin droplets from a blue bottle. Lockhart undergoes treatment inside a water tank.
The water fills up and the orderly tending to Lockhart gets distracted. The tank fills up with eels, causing him to panic. Lockhart rises and has his breathing tube pulled off by an eel. He nearly drowns until he gets the tank to open and spill the water out. Watkins relays the baron story to Lockhart when he brings it up to her. She mentions that the spa was built on the remains of his castle after the villagers burnt it down. Watkins says that the baby was cut out of the baron's sister's womb and then thrown into an aquifer, but it survived.
She also mentions that "She doesn't know... "Lockhart gives Hannah his ballerina figurine in exchange for a bike ride into town.
The two go together and stop at a local tavern. Lockhart buys himself and Hannah beers and runs into Enrico, who survived the crash and was given a new car with money that the spa gave him. Hannah stays to listen to music and do a dance by herself in front of the other patrons. Lockhart comes across a barn with a mute child who is drawing the castle fire. His father Pieter (Magnus Krepper) approaches Lockhart. Lockhart asks questions regarding the spa and its history.
Pieter then goes to a cow that is ailing and ready to die. He cuts the cow's stomach open, and out spills a stillborn calf and some eels. When Lockhart rejoins Hannah at the bar, he sees a punk dancing next to Hannah. Lockhart tries to get him away from her, but the punk attacks Lockhart. He is saved by Volmer, who arrives to pick up Lockhart and Hannah. Lockhart gets in touch with Green, who demands to know what happened with him and Pembroke. Lockhart mentions that Volmer called them about the accident, but Green knows nothing about the accident until Lockhart just mentioned it. He orders Lockhart to return to New York with Pembroke within the next 24 hours.
Lockhart's tooth comes loose and he must pull it out himself. He brings it to a staff member who places it in water. Lockhart then goes off to continue looking for answers. He sneaks past the staff as he goes through several wings. He comes across one room where several patients, including Pembroke, are kept inside water tanks, appearing dead. He also finds an underground area with a pool where dead bodies are dumped in and fed to eels.
One of those dead bodies happens to be Mrs. Watkins. When Lockhart tries to return to his room, he is found by the Caretaker (Johannes Krisch) along with Volmer and another staff member. Lockhart mentions the issue with his tooth. He is taken into a room and strapped down as he has a drill forced through his front tooth. Lockhart escapes the spa and heads into town to report Volmer's experiments to a police officer. Volmer and his staff member arrive to bring Lockhart back.
Lockhart mentions the tanks and how he saw Pembroke. Pembroke then emerges alive and well, backing up Volmer's claims that Lockhart threatened Pembroke and saying he would bring him back to New York by force if necessary. Lockhart has no choice but to return with Volmer. After undergoing more treatments, Lockhart starts to act and think like Pembroke in that he believes he is not well and must stay for a cure. He has a moment of clarity while writing a note to his employers where he thinks to himself that "no man can unsee the truth". Meanwhile, Hannah is using the pool and has her first period. Eels start swimming near her, but then they start swimming in a perfect circle around her.
Lockhart then breaks a glass and uses a shard to cut open his cast, revealing that his leg was never broken. He runs out of his room and finds Hannah. She is panicked and hits Lockhart in the face as he tries to help her. Hannah runs and finds Volmer in a room where he, the staff, and some patients are having dinner.
Lockhart runs in and starts to tell everyone that Volmer is a liar and is the one responsible for everyone being sick due to whatever is in the water, which is responsible for making their teeth fall out as a side effect. Patients start to stand, which Lockhart thinks is in support of him, but they are all actually going against him as they slowly approach him saying that they are unwell. They all crowd around Lockhart until he passes out. Lockhart wakes up in a chamber where he is immobile. Volmer forces a tube down his throat and uses the eel-filled liquid to put into his body where the eels filter out the "vitamins" that Volmer has all his patients take. Lockhart has his teeth fixed and appears to be changed just like the rest of the patients, stuck in a delusion that he is unwell.
Volmer throws a party that night with the patients and staff. He even gives Hannah a new dress for the occasion. In his room, Lockhart starts to come to his senses as he thinks about when Watkins told him about the baron. He finds a portrait that was taken a while after the fire. There is also a note saying "SHE DOESN'T KNOW". Lockhart breaks the portrait and uses a glass to magnify one subject in the picture - a man with bandages all over his face. What the portrait didn't show was the man holding hands with a little girl - Hannah. Volmer brings Hannah into a room by the transfusion wing.
There, he has kept a portrait of his sister, who was also Hannah's mother. Volmer ties Hannah to the bed and prepares to rape her. Lockhart breaks out of his room and first encounters the Caretaker, who attacks him until Lockhart causes steam to blow in his face before he bludgeons the Caretaker to death. Lockhart then finds Volmer and saves Hannah. Lockhart and Volmer fight, wherein Volmer pulls off the skin from his face to reveal himself as the hideously scarred baron. The "vitamins" have been keeping him and Hannah alive for well over a century. Lockhart leads Volmer into a trap where he dropped a bunch of fuel, leading Lockhart to set a fire. Volmer catches fire and then sets fire to the curtains as he tries to extinguish himself.
The fire then spreads up toward the rest of the castle. Lockhart tries to free Hannah, but Volmer attacks him. Volmer drags Lockhart near the eel pool and prepares to kill him, but Hannah takes a shovel and swings it down into Volmer's head. He stumbles backwards and falls into the pool to become eel food. Lockhart and Hannah escape just as the patients and staff flee the burning castle.
They take Hannah's bike and ride away. On the road, they are stopped by a car. Green, Hollis, and Wilson step out, ordering Lockhart to get in the car. When they ask about Pembroke, Lockhart says he is gone. They tell him to get in again, but he refuses. Hollis asks Lockhart what's wrong with him. Lockhart responds that he is feeling much better. He and Hannah continue riding away into the night, with a sinister grin forming on Lockhart's face.