Alex (Kathryn Newton) films her younger brother Wyatt's football match. At Alex's house, she shows her boyfriend Ben (Matt Shively) the treehouse in the garden. Here they discover Robbie, the neighbours child. Puzzled with why he is here she takes him back to his own house across the road. In the early house of the next morning Alex hears an ambulance outside at Robbie's house. The next day Robbie turns up to the house after Alex's mom offered to look after him for a few days because his mother went into hospital. Later on the day Alex finds Wyatt and Robbie in her treehouse talking to Robbie's invisible friend. In the evening Wyatt is playing multi-player on the Kinect by himself with Robbie sitting on the sofa.

Ben asks who Wyatt is playing with and Robbie tells him its his friend. When Alex arrives in the lounge Ben turns the lights off in the lounge and shows Wyatt through the camera the infra-red tracking dots. As Alex, Ben and Wyatt dance in the dark and Robbie sits on the sofa, the camera picks up something moving on the sofa next to him.

The next day Ben shows 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. He suggests that he can set up laptops around the house to record everything. Over the next couple of days strange occurrences happen around the house, such as Robbie waking up in the middle of the night and talking to the TV, Robbie and Wyatt running around the house in the early hours of the morning chasing a silhouette of a small child figure. 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). When alone in the house Alex hears noises and follows them to the hallway where the chandelier falls down from the ceiling and smashes into the floor.

Her father later blames this on the light fitters, but Alex is suspicious of Robbie. The next day she sees from her bedroom window that Robbie and Wyatt have ran across to his house. Alex follows them to discover that Robbie's mother is actually home, who is Katie (Katie Featherston). After Alex takes Wyatt back home he tells her that Katie told him he was adopted like Robbie. Alex questions her parents about Wyatt's real parents but they refuse to say anything. Wyatt later on then talks to an invisible figure, we only hear Wyatt's responses but Wyatt says that his name isn't Hunter. He later insists that his name is Hunter. At several points, Katie sneaks into the house to talk to Hunter, telling him she'll be with him until he's "ready." In the evening Wyatt appears in Katie's room and Alex is picked up into the air.

The next day when Alex's mother is downstairs alone in the house and starts to hear strange noises. Katie appears in the house and heads upstairs. When Alex's mother enters the lounge an unseen entity suddenly picks her up before dropping her onto the floor, instantly killing her. Katie comes back downstairs and drags her body off screen. Some time later Ben arrives at the house to discover no one is home. He goes to view the footage in Alex's bedroom but Katie appears and snaps his neck. Katie finds Wyatt in the house and tells him that he is Hunter.

Alex and her father arrive back home, 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 before. She runs out of the house and over to Katie's house to warn her dad who she witnesses 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. Just as she is about to run away with Wyatt, numerous blank-faced people appear in Katie's garden. Alex turns the other way only to be attacked by Katie before the camera cuts out.

http://www.imdb.com/title/tt2109184/synopsis