*CUT TO THE CHASE*
NOTE: This spoiler was submitted by Jeremy
The film opens with news coverage of the grisly crime scene at 112 Ocean Avenue in Amityville, New York. In 1974, Ronald DeFeo, Jr murdered his parents and siblings after claiming voices in the house made him do so. The news reporters wonder if there truly is evil living in that house.
Over forty years later, Joan Walker (Jennifer Jason Leigh) moves to the house with her children Belle (Bella Thorne), Juliet (Mckenna Grace), and James (Cameron Monaghan), and their dog Larry. James is brain-dead after an accident from a few years earlier. Joan thinks it's time for a fresh start in a new place, but Belle thinks nothing will change for the better regarding James's condition.
Belle starts at a new school. The other kids know about the house she lives in and they mock her for it. The only kid who is friendly with her is Terrence (Thomas Mann).
At night, Belle finds Juliet wandering the house. Belle takes her back upstairs to her room when they hear a beeping coming from James's room. Joan wakes up and runs to her son and tries to help him. James flatlines, but Joan apparently revives him.
Back at school, Belle does research on the house, learning about the murders that took place. Terrence approaches her and shows her a DVD of the original "Amityville Horror" film with James Brolin and Margot Kidder. He then suggests they watch it in the house, to which Belle agrees.
Joan meets with Dr. Milner (Kurtwood Smith) and her sister Candace (Jennifer Morrison) to discuss James's condition. Joan believes he really is getting better while Dr. Milner is more skeptical since others with James's condition have not usually come back. Belle is annoyed that everyone is continuing to put James through this. She leaves the room and finds Juliet talking to James, claiming he responds with things that only Belle would know.
Belle later has a nightmare that she finds the blood-stained wallpaper in her room before being face-to-face with Ronald DeFeo, Jr.
Terrence and his friend Marissa (Taylor Spreitler) arrive to watch the movie at 3:15 AM the following night, which is around the same time the murders happened. Terrence suggests the remake with Ryan Reynolds, but they decline (because "remakes suck"). The power goes out, so they go to the basement to find the fuse box. They find old things from previous owners, like the "High Hopes" sign. After the power comes back, Joan scolds Belle for having friends over without her permission.
Dr. Milner and Candace return to perform a test on James to see if he really has improved motor functions. Moments later, Dr. Milner experiences a vision of flies swarming all over him and James's body. He leaves the house freaked out.
Belle and Juliet experience more hauntings in the form of apparitions, shadows, and a dream in which James is fully mobile and he shoots Belle.
Candace brings a computer so that James may communicate with his eyelids. Words appear on the screen in response to questions the others ask him. Joan is elated that she can communicate with her son again.
Belle talks to Marissa about what happened to James. He went to defend her from punks that were tormenting her, and he was thrown from the second story, leading to his condition. Belle blames herself for what happened. Later on in the library, they meet with Terrence to discuss the possibility that James may be possessed by a spirit that is giving him improved function, and that the hauntings are due to a magic circle drawn around the house.
Belle goes to James's room to try and talk to him. When she asks how she can help him, he responds with "KILL ME" on the monitor, followed by "GET OUT". Belle pulls his breathing tubes out just as Joan walks in. She then sees that James is breathing on his own, making Joan think that he wants to live.
Terrence suggest to Belle that she find a way to prove to Joan that James is not who she believes he is. Belle uses the computer to write a message making it seem like James wants to go outside. Joan and Candace take him and Juliet out while Belle goes to the basement to find proof. She breaks down a wall to find "KILL THEM ALL" scratched into the bricks. Meanwhile, Juliet is spooked by a demonic reflection in the lake.
Belle tells her mom about the theory that James is possessed, even if it sounds crazy. Joan says she's not crazy and admits that she lost her faith in God after her husband died and James's accident, and she deliberately brought them to the Amityville house so that James could possibly be revived. Belle later tries to break out of the house with James, but Joan knocks her out.
Belle wakes up at 3:15 AM. She goes downstairs and sees Joan and Juliet seated for a meal, with James seemingly having prepared it. This, of course, turns out to be a dream. As she wakes up for real, a now fully-possessed James emerges from his bed, just as Candace shows up at the house. James goes down to the basement and retrieves a gun. He kills Candace and Larry, and then goes after Joan. She holds onto a crucifix to hold him back, but James shoots her dead. He goes to Juliet's room to kill her too, but Belle jumps and pushes herself and James out the window. She pulls him outside the circle, and the demon leaves James's body, reverting him back to his emaciated form. Before dying, he thanks Belle for saving him. She cries over her brother's body.
In the aftermath, Belle is questioned over the deaths of Joan, James, and Candace. Juliet is able to back up her story, and James's fingerprints are found on the murder weapon. Another news report comments on yet another tragedy occurring in the Amityville house.