The film unfolds with home video footage showcasing Kristi Rey, played by Sprague Grayden, joyfully bringing her newborn son Hunter, portrayed by William Juan Prieto and Jackson Xenia Prieto, back from the hospital. As we progress, a montage reveals Hunter's early months, allowing us to meet various family members including Kristi's husband Dan Rey, played by Brian Boland, and his daughter from a previous marriage, Ali, depicted by Molly Ephraim. The family also includes Martine, the housekeeper/nanny, and Abby, their loyal German Shepherd.
One year after Hunter's arrival, the family returns home to a perplexing scene: their house appears to have been ransacked despite the doors being locked, with only a necklace, a gift from Kristi’s sister Katie, portrayed by Katie Featherston, missing. In response, Dan installs security cameras throughout their home—including the backyard, baby’s room, kitchen, living room, entryway, and by the front door.
Disturbing events commence that first night: the pool light flickers ominously and dies, the pool cleaner is pulled from the water, and pots mysteriously topple in the kitchen. Hunter is frequently awakened by unsettling noises, while Abby reacts to unseen presences in his room. After one particularly troubling night, Martine attempts to cleanse the house of ominous spirits, but her efforts lead to Dan’s decision to fire her. It is revealed that as children, both Kristi and Katie struggled with the idea of being haunted by a demon, a past they had tried to bury.
Curiosity piqued, Ali probes into the strange occurrences and uncovers a chilling fact: making a deal with a demon comes at a grave cost—the life of the family's firstborn male. With Hunter being the first male born into Kristi and Katie's lineage in generations, tensions rise. Kristi confides in Katie about the unfolding horrors but is met with anger and a stern warning to ignore the events, fearing that acknowledgment would exacerbate the situation.
The supernatural violence escalates when Kristi encounters malicious forces in the kitchen, leading to her being dragged into the basement by an unseen entity, leaving her in a dazed state and seemingly possessed. Days later, Ali discovers Kristi in a disturbing state, marked by a bite on her thigh, while Hunter is in distress. A sequence unfolds when Chris and Ali conclude that they need to confront the menacing presence that has taken over Kristi. They call on Martine for assistance, who prepares a cross intended to expel the demon, with a foreboding caveat: the demon must be transferred to a blood relative to prevent it from returning.
In a harrowing climax, Dan attempts to remove the demon from Kristi, leading him to the basement in search of his family. The struggle culminates with a desperate attempt to place the cross on Kristi, unleashing chaotic supernatural phenomena. After the demon is apparently expelled, Dan makes a heart-wrenching decision to burn a photograph of young Katie, transferring the demon to her.
Fast forward three weeks, and Katie arrives at Kristi's, having begun to experience strange occurrences herself. Kristi, suffering from memory loss regarding the ordeal, innocently suggests that Katie dismiss the hauntings. The narrative shifts as Katie returns to her own home, where unsettling events from Paranormal Activity begin to unfold, eventually leading to the horrific murder of Micah, her partner.
As fate would have it, we witness a grim conclusion on the night after Micah's death, with Dan obliviously watching TV while Kristi tends to Hunter. Suddenly, Katie's haunting presence emerges, culminating in a brutal attack that leads to both Kristi and Hunter's demise. The film leaves viewers with a chilling denouement, as Katie walks away with the now nonverbal Hunter, leaving their fates shrouded in darkness. The final credits reveal that Ali, meanwhile, returns home from a school trip to find her father and stepmother dead, with Katie and Hunter's whereabouts unknown, setting the stage for ongoing horror.
