Brian and Kiera Woods relocate to an old, dilapidated house with their two kids, Ellie, a teenager, and Steven, a young elementary student. Right from the start, Ellie expresses her discontent, wishing instead to be with her friends.
On their first night, Brian and Kiera leave Ellie in charge of Steven while they head into the office for work, much to her annoyance. While exploring, Steven discovers a hidden room filled with old artifacts and starts to play a peculiar device called the sonographe. The device projects a man's voice that begins with formulas and slowly counts upward. Ellie, sensing an eerie presence, switches it off before it can reach seven and tells Steven to go to sleep.
That night, as Ellie sits watching TV and the power unexpectedly goes out, she frantically calls her mother. Kiera instructs her to count to ten, which corresponds to the number of steps leading down to the cellar where the breaker is located. However, when Ellie reaches ten, she begins to count in a mesmerizing and dreamlike cadence. Alarmed, Kiera and Brian rush home only to discover that Ellie has vanished, prompting a massive search effort.
The following day, with Ellie still missing, the local police conclude that she has run away, given her previous history of doing so. Yet, Kiera has an unsettling feeling that something much more sinister has transpired, so she embarks on her own investigation. In the cellar, she uncovers mysterious equations etched into the floor along with haunting, plant-derived paint depicting faces in red. After a glance, the police identify the markings as belonging to the 1950s, and Brian admits they secured the property at auction from an elderly woman for a mere pittance.
Later that night, after helping Steven with a bedwetting accident, Kiera hears unsettling whispers emanating from the upstairs bathroom sink in the form of a young girl's counting. Chillingly, as the counting continues, it lures Kiera toward the cellar. Despite turning on the light and venturing down, she finds naught but silence.
The next day, Kiera questions Brian about the peculiar symbols above the doors throughout their new home. She takes photos of these symbols and seeks the help of her coworker, Erica, to decipher their meanings. Erica reveals that the glyphs are Hebrew, spelling "Leviathen," the name of a legendary sea creature from Jewish lore.
In the following events, Kiera plays the sonographe again, this time allowing Steven to count along. However, he becomes entranced and moves toward the secret room. Kiera snaps him back to reality. While researching a painting of a previous occupant, John Fetherston, the cellar door inexplicably opens, and the lights flicker out. Steven cries for help from within the cellar, causing Kiera to rush toward the noise. Looking through the keyhole, she sees a monstrous eye staring back but is taken aback when Steven, fast asleep, asks her who she is speaking to. As the door creaks open, the power returns.
Kiera descends into the cellar, only for the lights to extinguish once again as the door slams shut behind her. A growling sound ascends the stairs after Kiera accidentally drops her phone. In a moment of terror, Brian bursts in to save her. Kiera insists that a malevolent presence lurks below, but Brian, skeptical, goes down himself and finds nothing but animal fur, her phone having vanished.
The next day, Kiera visits the National College of Mathematics to meet Dr. Remi Fournet. After analyzing the strange equations she discovered, he informs her that the phrase refers to complex dimensions and is a remnant of a theory associated with Erwin Schroödinger. Remi shares that Fetherston was previously involved with him before mysteriously disappearing after his son fell ill, further raising Kiera's alarm.
First Brian dismisses Kiera's growing theories, but eventually accompanies her to explore the cellar symbols. In a moment of misunderstanding, Steven drifts toward the mysterious secret room once again. Brian accuses Kiera of hallucinating, and in frustration, attempts to destroy the bottom step but when he does, the sonographe starts counting ominously again. In a horrifying vision, Steven sees a twisted version of Ellie with eerie symbols etched onto her face chanting along. After a scream, they retrace their steps upstairs, where Brian discovers Ellie's phone in the secret room.
Kiera seeks out Rose Fetherston at St. Anthony's nursing home. Rose reveals that it was her father who unleashed the dark entity to save her ailing brother, Jack, naming it Leviathen, one of the seven princes of hell. She warns Kiera that the evil is not confined to the cellar—its presence envelops the entire house.
Returning with newfound urgency, Brian reveals his research indicating that the symbols line up to form a pentagon, and combined with Leviathen, they invoke the name "Baphomet," a goat-human figure adorned with black wings, revered by the occult knights of history. As tension mounts, Steven inadvertently follows his drone into the cellar where the sonographe begins to play again. In this moment of horror, Baphomet appears, capturing Steven as the lights flicker out, leaving Kiera and Brian in frantic search.
With the power failing, they find Steven in the secret room, where he bears a carving on his chest and eerily begins counting down from 48. As Kiera runs from Baphomet, they descend a seemingly endless staircase toward a lone door at its end. Kiera hides behind the door, narrowly evading Baphomet. Pushing forward, she finds a surreal crowd of people, all counting in unison, trapped in a time loop.
Just as despair threatens to sink in, Kiera discovers Ellie and brings her back, confronting Baphomet with defiance, declaring that he cannot claim her family. United at last with Brian and Steven, Kiera leads her loved ones to the front door, only to reveal they are still ensnared in the demon's dimension. In an eerie twist, as they begin counting upward, Ellie, Steven, and Brian drift into the cellar once more, with Kiera eerily joining their count as the credits roll.
