Alex, portrayed by Kathryn Newton, captures her younger brother Wyatt's football game on camera. Later, at her home, she shares with her boyfriend Ben, played by Matt Shively, the joy of the treehouse nestled in their garden. Intriguingly, they stumble upon Robbie, a young boy from the neighborhood, and Alex is left wondering why he is there. Concerned, she takes him back to his home across the street.
As dawn breaks the following day, Alex hears the unsettling sound of an ambulance outside Robbie's residence. Soon after, Robbie arrives at Alex's place, as her mother had generously offered to look after him during his mother's hospital stay. On that very day, while exploring the treehouse, Alex discovers Wyatt engaged in a conversation with Robbie's invisible friend.
In the evening, as Wyatt is engrossed in a multiplayer game on the Kinect, Robbie joins him on the sofa. When Ben inquires about Wyatt's playmate, Robbie innocently claims it's just his friend. Alex enters the lounge, only to find Ben turning off the lights to reveal the fascinating infrared tracking dots on the camera. As the three of them dance in the dark, a moment of eeriness occurs when the camera captures something shifting next to Robbie on the sofa.
The subsequent day, Ben shows Alex a peculiar feature of his computer: it records their webcam sessions automatically. An unsettling moment arises when they discover that Robbie had unknowingly entered Alex's room at night and snuggled next to her while she slept. Ben suggests setting up additional laptops throughout the house to monitor their surroundings.
As the days roll on, bizarre happenings escalate in the house. Robbie wakes up in the middle of the night, conversing with the television, and he and Wyatt are seen sprinting around the apartment chasing the shadow of a small child. An alarming scene unfolds when Robbie draws a strange symbol on Wyatt's back, which Alex later learns is linked to an old cult referenced in Paranormal Activity 3. As Alex explores the house alone one evening, she is startled by the chandelier crashing to the ground. Although her father attributes the incident to faulty workmen, Alex's suspicion of Robbie grows.
Curiosity leads Alex to spot Robbie and Wyatt racing over to his house. When she follows, she uncovers the shocking truth—Robbie's mother, Katie (played by Katie Featherston), is actually home. After retrieving Wyatt, he confides in Alex that Katie claimed he was adopted, just like Robbie. As the story unfolds, an invisible figure interacts with Wyatt, who insists that he isn't actually named Hunter, although he later claims that it is his name.
Throughout various scenes, Katie stealthily enters the house to speak with Hunter, assuring him that she will stay with him until he feels "ready." One harrowing evening, Wyatt finds himself in Katie's room, while Alex is inexplicably lifted into the air.
The next day, while Alex's mother is alone at home, unsettling noises fill the air. Katie materializes upstairs, leading to a shocking moment when an unseen force violently lifts and drops Alex's mother, causing her immediate death. Katie, exhibiting a chilling demeanor, hauls the body away without a trace.
Before long, Ben visits and finds the house deserted. He decides to review the surveillance footage in Alex's bedroom, but he is swiftly confronted by Katie, who sadly snaps his neck. Katie eventually locates Wyatt, revealing to him that he is indeed Hunter.
When Alex and her father return home, he mistakenly glimpses Alex's mother alongside Wyatt, walking towards Katie's abode. In an effort to investigate, he separates from Alex, who discovers Ben's lifeless body. In a panic, Alex races to Katie's house to warn her father, only to witness him being yanked violently by an unseen force. In desperate need of safety, she retreats into a distant bedroom, only to find it empty. As she turns back, Katie appears unexpectedly, screaming at her with an otherworldly fury.
In a heart-pounding escape, Alex leaps through the window, only to find Wyatt waiting outside. Just as they attempt to flee, they are confronted by a chilling group of blank-faced figures assembling in Katie's garden. In a final twist of terror, Katie attacks Alex, and the screen suddenly fades to black, leaving audiences clinging to the edge of their seats.
