Cousins Teresa and Jessica, along with Jessica's new boyfriend Caleb, are spending their vacation at a serene lakeside cabin when a shocking incident occurs. An apparently disturbed Jessica suddenly scalps Teresa and decapitates Caleb, all while levitating ominously above the water. This startling sequence of events comes right after Beth, a guitar technician and sister to Ellie, visits her sibling in Los Angeles. Despite being a single mother, Ellie, played by Alyssa Sutherland, is doing her best to raise her teenagers Danny and Bridget, along with young Kassie.
Their lives take a dark turn when an earthquake rattles their condemned apartment complex, revealing a hidden chamber in the basement. Inside, Danny discovers religious artifacts and three phonograph records from 1923, along with a peculiar book he believes could help his struggling mom. The first record details the failed attempts of a priest to investigate the book, which turns out to be one of three volumes of the Naturom Demonto. The second record exposes the priest's secret research and an incantation that conjures demonic entities known as Deadites.
As the power fails in the building, Ellie becomes possessed by a sinister unseen force. In a trance, she endangers her family, pleading for Beth to protect her kids before ultimately succumbing to her possession. With the main exit routes blocked—thanks to the earthquake—the family finds themselves trapped, forcing them to confront the horrors that lurk within.
In a series of horrifying events, Beth and the children manage to lock Ellie outside after witnessing the brutal massacre of their neighbors. Unfortunately, the evil spreads further as Danny confesses about the Naturom Demonto, and Ellie deceives Kassie into unlocking the door. The nightmare escalates as Bridget is soon possessed too, leading to a chaotic confrontation that results in her death at the hands of Kassie, who inadvertently impales her with a broken broom handle.
Desperate for a solution, Beth listens to the last record, only to learn that the priest's attempts to combat the Deadites were in vain, as total destruction of the host is the only way to stop them. The possessed Bridget fatally wounds Danny, who in a last-ditch effort, sets her ablaze, just as Ellie invades the apartment through the vents.
When Ellie realizes Beth is pregnant, she tries to endanger the fetus, but Beth and Kassie manage to subdue her temporarily. In the midst of rising terror, they take refuge in a damaged elevator as Ellie, Bridget, and Danny merge into a grotesque creature known as the Marauder. This monstrosity attacks them, spilling blood within the elevator until it crashes to the ground floor.
In the parking lot, Kassie is captured, and Beth courageously returns to save her. Together, they cleverly dispose of the Marauder by overpowering it with a wood chipper, ensuring Ellie's decapitated head is no longer a threat as it mocks Beth about her motherly capabilities. The duo escapes the nightmarish complex, although their harrowing ordeal is far from over, as the following morning reveals an unseen terror attacking Jessica in the parking lot as she prepares to leave for her vacation.
