Ben and Tina, a young engaged couple from New York and enthusiastic YouTube content creators, embark on an adventurous journey across Europe in search of reputedly haunted locations to document their thrilling exploits. One day, they make their way to south-west France, aiming to explore a sanatorium submerged beneath an artificial lake. However, to their surprise, they find the site bustling with vacationers. Thankfully, a local named Pierre offers to guide them to a secluded part of the lake, deep in the forest of Chanteloup. This area, which was intentionally flooded in 1984 to mitigate the risk of devastating floods, is rumored to house a mansion that, according to Pierre, has remained astonishingly intact.
Upon arrival, Ben and Tina dive into the water and quickly locate the mansion, discovering its contents eerily well-preserved. Yet, once they enter the house, a series of disturbing events unfold. They begin to hear unsettling voices and noises, while their drone's motion tracker picks up inexplicable movement. Their electronic devices start malfunctioning mysteriously, adding to their growing sense of unease. As they venture further into the mansion, they stumble upon numerous photographs, posters, and newspaper articles of missing children, alongside satanic symbols and ominous scratches on the front door.
In the kitchen, they encounter a doorway blocked by a large crucifix. When they manage to open it, they are horrified to find two corpses restrained in chains, adorned with torture masks, hanging above a satanic pentagram, with a nearby room filled with pickled human remains. In a desperate attempt to escape, they discover that the window through which they entered has been sealed with a brick wall, rendering their search for alternate exits futile.
During a tense moment when Tina attempts to pry open a grate in the cellar, she is suddenly attacked. Strangely, the assault stops abruptly, and to Tina's bewilderment, Ben asserts that nothing out of the ordinary has occurred. Intrigued, Ben uncovers the masks from the corpses, revealing them to be the Montégnac family, the original owners of the house. Unexpectedly, the corpses animate, chasing them through the eerie halls. As they try to escape through a chimney, it collapses, separating them on different levels of the house.
In an upstairs chamber, Ben discovers a family tree, which uncovers Pierre's true identity as the son of the Montégnacs, hinting that he deliberately lured them into the haunted dwelling. Shortly after, he becomes possessed by the undead daughter, Sarah. When Tina finds him, he leads her to a secret room in the basement, where Sarah, controlling Ben, shares a gruesome film reel. This chilling footage reveals that Mr. Montégnac and Pierre kidnapped local children for sinister sacrifices. An avenging mob ultimately killed the elder Montégnacs and young Sarah, yet Pierre managed to evade their wrath.
Under Sarah's influence, Ben tries to persuade Tina to join their twisted family dynamics. Frightened and gasping for air, Tina escapes to a hidden satanic chapel and discovers a shaft leading out. Ben catches up to her, attempting to take her life, but in self-defense, she strikes him with a diving knife, breaking him free from the grip of possession. Tragically, before they can make their escape, Sarah fatally stabs Ben. With the Montégnacs turning their malevolent gaze towards Tina, she narrowly escapes through the shaft leading back to the lake. However, as she reaches the surface, her lungs give out, and she succumbs to drowning.
In a haunting epilogue, the camera reveals two new divers peering across the lake, accompanied by none other than Pierre, hinting at the chilling cycle that may continue.
