As Lockhart (Dane DeHaan), a driven executive from New York City, arrives at a stunning wellness center in the Swiss Alps to bring back CEO Roland Pembroke (Jason Isaacs), he faces firm resistance from the staff and the cryptic Dr. Heinreich Volmer. Ordered by the board of directors to retrieve Pembroke, who unexpectedly checked into the center, Lockhart's efforts to communicate with the CEO are continuously thwarted.
Frustrated, Lockhart exits the center but soon gets into a minor car accident, resulting in a plaster cast for his leg. When he wakes up three days later, he is bewildered to find himself back at the wellness center, now as a patient. Surprisingly, both he and the other driver escaped with only minor injuries.
As he navigates the complex and winding hallways of the facility, Lockhart meets a variety of intriguing individuals, including Hannah, a secretive young girl harboring her own mysteries. Among the other patients is Victoria Watkins, whose fascination with the facility's grim past ignites Lockhart's curiosity. The locals share eerie tales about the spa's troubled history, which stretches back to the 18th century.
Local legends state that the spa was constructed on the ruins of a castle once belonging to a relentless baron who desperately wanted an heir of pure blood. In a shocking turn, he married his sister, forcing the local peasants to endure horrific experiments in his search for a cure for her infertility. Though his methods yielded results, it came at a dreadful price.
As Lockhart digs deeper into the enigmas of the spa, an intense desire to escape grows within him. Nonetheless, every effort meets with unexpected obstruction from the staff, who appear determined to keep their patients confined. After Lockhart bestows a delicate ballerina figurine to Hannah, she offers him her assistance in maneuvering the intricate network of tunnels and corridors.
Together, they embark on a quest to unveil the truth behind Pembroke’s enigmatic disappearance. Lockhart's journey leads him to the heart of the nearby town, where he uncovers a mystery: the townsfolk are suffering from dehydration, despite having access to the aquifer's water. As he seeks to uncover answers, Hannah draws unwanted attention in a local bar, inciting the anger of the townspeople.
During a tense altercation, Lockhart defends Hannah, only to be rescued by Dr. Volmer, whose timely intervention seems to intimidate the locals into silence. As the tension escalates, Lockhart finds himself ensnared in a web of deception and secrets that threaten to engulf him.
While Lockhart explores the spa's captivating secrets, a disturbing reality emerges: the therapeutic waters of the aquifer hide a sinister truth—its toxic properties from the eels thriving within. The baron’s malevolent genius orchestrated a method to extract this essential essence by passing it through human bodies, with Volmer's patients serving as uninformed vessels. Soon, Lockhart becomes aware that his leg is notbroken and that he is being held captive, subjected to intense psychological torment that gradually unravels his sanity.
In the meantime, Hannah, once seen as a delicate figure, undergoes a significant transformation after experiencing the "cure." Alongside Volmer and his staff, she now embodies an unnatural longevity. Just as Lockhart teeters on the brink of madness, Hannah’s gentle touch brings him back to reality when she returns the ballerina figurine, breaking the spell of his delirium.
As miraculous changes begin to manifest, Hannah experiences her first menstruation—an emotional milestone. Driven by an insatiable hunger for power, Volmer marries Hannah in a grotesquely twisted ceremony. The night ultimately spiral into horror when Volmer, concealed behind his disfigured visage, attempts to assault his daughter within the castle's hidden chambers.
Fueled by this shocking revelation, Lockhart faces off against Volmer in a confrontation that unravels a shared truth. A savage struggle ensues, and at the apex of their battle, Lockhart finds himself overpowered by the monstrous baron. Yet, with unwavering determination, Hannah strikes down her father, sending him tumbling into the aquifer to meet a fate among the very eels that had sustained his unnatural life.
As the castle collapses around them, Lockhart and Hannah make a narrow escape on her bicycle, colliding with a car containing Lockhart's employers. The fate of Pembroke remains a haunting question. With an eerie sense of tranquility, Lockhart declares Pembroke's death, brushing aside their pleas as he rides off with Hannah, lost in deep contemplation, his face illuminated by a newfound feeling of liberation.
