Alex Nelson films her younger brother, Hunter Rey/Wyatt's, soccer match. At Alex's house, she shows her boyfriend Ben the tree house in the garden. Here they discover Robbie, the neighbor's child. Puzzled as to why he is there, she takes him back to his own house across the street. In the early hours of the next morning, Alex hears an ambulance outside at Robbie's house. That day, Robbie turns up at the house after Alex's mom, Holly Nelson, offered to look after him for a few days, while Robbie's mother is admitted to hospital. Later on, Alex finds Wyatt and Robbie in her treehouse talking to Robbie's invisible friend who Robbie calls Toby.
In the evening, Wyatt plays multi-player on the Kinect by himself, while Robbie sits on the sofa. Ben asks who Wyatt is playing with and Robbie answers that it's the invisible friend from the tree house. Ben turns the lights off and shows Wyatt the infra-red tracking dots. As Alex, Ben and Wyatt dance in the dark, Robbie sits on the sofa, where the camera detects an unknown figure moving next to him.
The next day Ben reveals to Alex that his computer automatically records their webcam chats, and in the middle of the night while Alex was sleeping, Robbie came in and slept next to her. Baffled by his odd behavior, Ben offers to install laptops around the house to record any other strange occurrences. Over the next couple of days, the computer records strange events around the house, including Robbie waking up in the middle of the night to talk to the TV, and Robbie and Wyatt chasing the silhouette of a small child figure throughout the house. Robbie draws a strange symbol on Wyatt's back, which Alex later finds to be part of an old cult (first appeared in Paranormal Activity 3).
Late one night Alex notices several black cars outside Robbie's house across the street; upon investigating, she discovers several women in black garb entering the house. She is confronted by one of these women and flees the scene. The next day Alex is home alone, and after hearing noises, follows them to the hallway, where the chandelier falls from the ceiling and smashes into the floor, almost striking her. Although her father Doug Nelson blames the light fitters, Alex is suspicious of Robbie.
The following day, Alex sees Robbie and Wyatt entering Robbie's house across the street. Alex follows them to discover that Robbie's mother Katie Featherston is actually home, and shows no sign of illness. Wyatt tells Alex that Katie told him he was adopted, like Robbie. Alex questions her parents about Wyatt's real parents, but they refuse to say anything. Later that night, Wyatt begins talking to an invisible figure, insisting that his name isn't Hunter. During this conversation a figure approaches him, but is interrupted when Doug enters the room.
The next day, Alex is home alone talking with Ben on the computer when suddenly the garage door opens. She goes to the garage and closes the door and goes back to the kitchen when the garage door opens again. She goes back to the garage and suddenly the garage door slams shut and so does the other door and the car turns on, filling the garage with Carbon Dioxide. Meanwhile, Wyatt is hiding in the bathroom connected to his room as Katie walks in and asks him to come out, using the name Hunter. She then tells him that she'll wait for him until he's ready. Meanwhile, in the garage, Alex picks up a golf club and smashes a window in the car, goes into the car and drives it through the garage door, giving herself fresh air. Her parents come and aren't convinced by her story.
Later that same day, Holly takes Wyatt for a bath and leaves him alone when he is violently pulled under and after a couple minutes he comes back up calm and sedated. That night, Wyatt walks into Alex's room and watches as she levitates above the ground.
The next day, Holly is alone, and begins to hear strange noises. A possessed Katie enters the house undetected, and heads upstairs. When Holly enters the lounge an unseen entity suddenly picks her up and violently throws her against the walls, before dropping her on the floor. Katie returns to drag her body away. Ben later arrives to discover no one is home. He goes to view the footage in Alex's bedroom, but Katie appears behind him and snaps his neck.
Alex and her father arrive, and he thinks he sees Alex's mother with Wyatt walking to Katie's house. As he goes to explore Alex goes home to discover Ben's body. She runs out of the house and over to Katie's house to warn her dad, but witnesses him being dragged violently through the house by the unseen entity. She runs to a bedroom at the far end of the house but can't find his body. As she turns around to head back out, Katie appears who storms forwards, demonically screaming to Alex. Alex quickly runs and climbs out of the window where she discovers Wyatt standing outside. She pleads with him to run away with her, but he stands there, blank faced and unmoved, just as numerous blank-faced people appear in Katie's garden. Alex turns the other way only to be attacked by a screaming Katie before the camera cuts out. captures the paranormal lift up paranormal green jacket t rex super tough duct tape wrapping around the paranormal head put sack on the paranormal head and body head arm legs tail put paranormal feet on converse shoes saran wrap wrapping around the paranormal head make into a t rex unstoppable form paranormal head and body all gone
- Post-credit ending