Cousins Teresa and Jessica, and Jessica’s new boyfriend Caleb, are vacationing at a lakeside cabin when a seemingly ill Jessica scalps Teresa and decapitates Caleb before levitating above the lake. One day earlier, upset at learning she is pregnant, guitar technician Beth visits her sister Ellie, a tattoo artist and single mother to teenagers Danny and Bridget, and child Kassie, in their home at Monde Apartments, a condemned Los Angeles apartment complex. The building is shaken by an earthquake while the children are in its basement parking lot, uncovering a concealed chamber. Danny investigates it, discovering religious artifacts, three phonograph records from 1923, and a strange book that he takes up to his room, believing he could possibly sell it and use the money to help Ellie. The initial record details rejected efforts by a priest to research the book, revealed to be one of three volumes of the Naturom Demonto. The subsequent record reveals the priest continued his research in secret and recites an incantation that summons demonic entities known as Deadites. The building’s power fails, and an isolated Ellie is attacked and possessed by an unseen force. She returns to the apartment in a trance, menacingly threatens her family, and dies after pleading with Beth to protect her children. Beth and Ellie’s neighbors help lay her to rest in her bedroom and search for a way out. They find that the staircase has collapsed, the elevator is damaged, and they are unable to access the fire escape before Ellie revives and attacks the family, wounding Bridget. Beth and the children lock Ellie outside the apartment after she pursues and massacres the neighbors. Danny confesses to Beth about recovering the Naturom Demonto, and Ellie tricks Kassie into unlocking the door before attacking her. While Danny and Beth are distracted rescuing Kassie and locking Ellie out, Bridget becomes possessed via her wound. Bridget attacks Beth before turning on Danny and Kassie, who inadvertently impales her through the head with a broken broom handle. Beth listens to the third record to understand how to exorcise the Deadites, but learns the priest failed and his allies were all possessed, with only complete destruction of the host stopping the Deadites. Bridget revives and fatally stabs Danny, who sets Bridget on fire before dying as Ellie infiltrates the apartment using the vents. Realizing Beth is pregnant, Ellie attempts to rip the fetus out of her, but Beth and Kassie are able to incapacitate her with scissors. Ellie fails to emotionally manipulate Kassie who accepts that her mother is already gone. Danny and the bodies of the neighbors are possessed, leading Beth and Kassie to take shelter in the damaged elevator. Ellie, Bridget, and Danny merge into a multi-limbed creature known as the Marauder and climb atop the elevator to attack the pair as the machine fills with blood. The combined weight causes the elevator to plummet to the ground floor, allowing Beth and Kassie to flee into the parking lot. The Marauder captures Kassie and attempts to behead her with a chainsaw, but Beth returns and distracts it, and she and Kassie destroy the Marauder’s body by forcing it into a wood chipper. Ellie’s decapitated head taunts Beth, saying she will be a failure as a mother, prompting her to kick Ellie’s head into the chipper. Beth and Kassie escape the building together. The next morning, Jessica goes to the parking lot to leave for her vacation, where she is attacked by an unseen force.
