Ben and Tina, a lively young couple from New York, are engaged and dive enthusiastically into their passion for YouTube content creation, where they explore infamous haunted locations. Their adventures lead them to the picturesque countryside of south-west France, in search of a notorious sanatorium that now lies beneath an artificial lake. Unfortunately, upon arrival, they discover that the site has transformed into a bustling vacation destination.
Their fortunes change when a local named Pierre offers to guide them to a secluded section of the lake, deep within the forest of Chanteloup. This area was submerged in 1984 in an effort to halt relentless flooding. Pierre claims that within this forest lies a mansion that has remained “perfectly preserved” over the years.
As Ben and Tina dive into these mysterious waters, they quickly find the mansion, surprisingly intact yet shrouded in an ominous atmosphere. Upon entering the house, the couple is engulfed in a series of unsettling events. Disturbing voices echo through the halls, their drone's motion tracker reacts to unseen presences, and their electronic devices falter mysteriously. They stumble upon rooms filled with chilling photographs, news articles of missing children, and cryptic satanic symbols. One door, ominously blocked by a large crucifix, leads to a horrifying room where they find two corpses shackled and enshrined in grotesque masks amid a satanic pentagram, accompanied by disturbing jars containing preserved body parts.
In a frantic attempt to escape, the couple discovers that the window they entered has been sealed shut by a sudden brick wall, trapping them inside the mansion of horrors. An attempt to pry open a grate in the cellar results in a shocking attack on Tina, who witnesses a horrific series of events that Ben cannot acknowledge, leading to a rift between them. Upon removing the masks from the corpses, Ben discovers them to belong to the Montégnac family, the former owners of the house. To their horror, these corpses animate, pursuing the couple through the progressively hostile house.
As they seek to flee via the chimney, the structure collapses, separating them. In the bedroom, Ben uncovers a family tree that reveals Pierre as the son of the Montégnacs, suggesting he has ensnared them into this trap deliberately. In a shocking twist, Ben encounters the undead daughter, Sarah, who possesses him, further complicating their plight. Tina, distraught, uncovers a hidden room where Sarah reveals the family's dark history of kidnapping children for satanic rituals, culminating in a gruesome demise at the hands of a vigilante mob.
Controlled by Sarah's malevolent intent, Ben attempts to draw Tina into the Montégnac fold. Desperately seeking an escape, Tina races into a secretive chapel and discovers an exit that leads out. However, chaos ensues when Ben, momentarily freed from possession, turns against her. In a heart-pounding struggle, Tina manages to wound him, only for the conflict to end tragically as Sarah fatally stabs Ben.
As Tina escapes through the opening leading back into the lake, she faces her ultimate challenge; she surfaces only to realize she’s out of air and tragically drowns beneath the waters.
In a chilling post-credit scene, two divers peer into the lake, their presence unsettlingly reminiscent of the cycle of terror that continues, altered now by Pierre’s guiding hand.
