The movie opens with a gruesome prologue at a lakeside cabin where a possessed teenager attacks her cousins, setting up the grim tone. The story then rewinds one day earlier to Los Angeles, following guitar technician Beth as she discovers she is pregnant. Seeking comfort, Beth visits her estranged older sister Ellie, a tattoo artist raising three children - Danny, Bridget, and Kassie - in a condemned apartment complex.

While the kids are sent out to grab pizza, an earthquake hits the city, opening a hidden bank vault underneath the apartment building's parking lot. Curiosity gets the better of Danny, who explores the hole and discovers bizarre religious artifacts, strange vinyl records, and a skin-bound book locked with fangs. Danny opens the book, the Naturom Demonto, and plays the vinyl records in his bedroom. The recordings play a priest reciting ancient incantations, which unleashes the demonic force lurking in the earth.

The entity stalks into the apartment elevator and viciously attacks Ellie, wrapping her in cables and violating her soul. A dazed Ellie returns to the apartment, only to violently convulse, vomit bile, and collapse after warning her sister to protect the children. When Ellie dies, the sisters and neighbors try to call for help, but the stairs are collapsed and the elevator is broken.

Ellie returns to life as a Deadite, immediately slaughtering her neighbors in the hallway and hunting her own children. Bridget is scratched and slowly turns into a monster, swallowing glass and attacking Danny. Beth is forced to fight through the night to protect young Kassie as her sister's demon mocks their family trauma. Danny tries to burn the remains of Bridget and dies in the struggle, leaving only Beth and Kassie alive.

The Deadite family fuses together into a massive, multi-limbed creature known as the Marauder. Beth and Kassie escape down a blood-filled elevator shaft to the underground parking garage. Beth arms herself with a chainsaw and battles the Marauder, eventually feeding the monster head-first into an industrial wood chipper. Beth and Kassie escape into the morning light just before Jessica, a resident going on a lakeside vacation, arrives in the garage and gets possessed, looping back to the film's opening scene.

The climax ties the standalone apartment story directly back to the prologue at the lake. Beth uses a chainsaw and a wood chipper to grind the Marauder into red mist, grabbing Kassie and driving away to freedom as a newly formed mother figure. Moments later, neighboring resident Jessica walks into the bloody garage, completely unaware of the massacre. The lingering demonic spirit attacks and infects Jessica, who then drives out to the cabin at the lake to trigger the prologue seen at the start of the movie.

This structure confirms that the book discovered in the vault is one of the three volumes of the Necronomicon mentioned in Army of Darkness, proving multiple books exist across different locations in the same universe.