The film begins with intense news coverage surrounding the horrific events that transpired at 112 Ocean Avenue in Amityville, Long Island, New York. In 1974, a young man named Ronald DeFeo, Jr. brutally killed his entire family, claiming that he was influenced by chilling voices echoing through the house. The reporters grapple with the sinister presence that seems to linger in the infamous residence.
Fast forward over forty years later, Joan Walker (Jennifer Jason Leigh) relocates to this very house with her children: Belle (Bella Thorne), Juliet (Mckenna Grace), and James (Cameron Monaghan), alongside their dog, Larry. Following a tragic accident that left James brain-dead years ago, Joan yearns for a fresh start in this new environment. However, Belle, sensing no hope for a change in her brother’s condition, remains skeptical.
As Belle adjusts to her new school, her peers are aware of her living situation and subject her to ridicule. The only student offering her any kindness is Terrence (Thomas Mann). One night, Belle finds Juliet wandering the hallways aimlessly, and as they return to her room, an alarming beeping from James’s room draws their attention. Joan rushes in, desperately trying to revive him as he flat-lines, but miraculously, she brings him back.
While at school, Belle conducts research about the ominous history of the house, uncovering the tragic events that unfolded there. Terrence introduces her to a DVD of the classic "Amityville Horror," featuring James Brolin and Margot Kidder. Enthusiastically, Belle agrees to watch it with him at the house, setting the stage for eerie events to follow.
During a meeting with Dr. Milner (Kurtwood Smith) and her sister Candace (Jennifer Morrison), Joan expresses her conviction that James is improving, but Dr. Milner harbors concerns based on his experiences with similar cases. Frustrated by her family's hopefulness regarding James, Belle storms out and discovers Juliet engaged in a conversation with James, wherein she insists he is responding with knowledge only Belle possesses.
Belle eventually experiences a terrifying nightmare involving blood-stained wallpaper and a haunting encounter with Ronald DeFeo, Jr. That following night, Terrence, along with his friend Marissa (Taylor Spreitler), arrives to watch the movie at the stroke of 3:15 AM, echoing the time the infamous murders took place. As the power unexpectedly cuts out, they venture into the basement to locate the fuse box, unearthing remnants from prior owners, like an ominous "High Hopes" sign. Once the lights flicker back on, Joan scolds Belle for having friends over without her consent.
As Dr. Milner and Candace conduct tests on James, Dr. Milner suddenly experiences a disturbing vision, causing him to leave the house in haste. Meanwhile, Belle and Juliet confront further hauntings characterized by shadowy figures and unsettling dreams in which James is able to walk only to aim a gun at Belle.
Candace arrives with a computer designed for James to communicate using his eyelids, allowing words to appear on-screen while Belle confides in Marissa about the tragedy surrounding her brother's injury. Belle learns that James was injured in an attempt to protect her from bullies and struggles with the guilt of that night. The trio begins to suspect that James may be possessed and that the house's dark history feeds this supernatural unrest.
In a moment of desperation, Belle seeks to engage James, only to be met with chilling messages on the computer, “KILL ME” followed by “GET OUT.” In the midst of this, Joan appears, believing that James truly wishes to live as she witnesses him regain some strength. Terrence then urges Belle to expose the truth about James's possession.
Determined, Belle heads into the basement, where she finds a chilling message scratched into the wall: “KILL THEM ALL.” At the same time, Juliet encounters a sinister sight in the lake. Belle reveals her fears of James being possessed to her mother. Joan, philosophical and shaken, recounts her declining faith in God following her husband's death and the trauma surrounding James, which led her to make a fateful decision in moving to the Amityville house in search of a miracle.
Later, as Belle plans to escape the house with James, Joan confronts her and unintentionally renders her unconscious. Upon awakening at 3:15 AM, Belle observes Joan and Juliet at a table, enjoying what appears to be a meal prepared by James—only to realize it was an illusory dream. Upon fully regaining consciousness, she witnesses James, now entirely overtaken by possession, rise from his bed, just as Candace arrives at the house.
In a whirlwind of chaos, James retrieves a firearm from the basement and, haunted by the looming evil, kills Candace and Larry before turning his sights on Joan. Armed with a crucifix, Joan attempts to ward him off, but tragically, James succumbs fully to the darkness and shoots her. As he approaches Juliet, Belle instinctively intervenes, launching herself and James through the window.
Upon falling outside, the dark presence leaves James's body, reverting him to his frail state. In his final moments, he expresses gratitude to Belle for saving him, leaving her in tears beside him.
In the aftermath of this tragic night, Belle faces a police investigation concerning the deaths of Joan, James, and Candace. Despite the terror they've endured, Juliet supports Belle’s account, and evidence against James surfaces, pointing to the tragic legacy of the haunted Amityville house, which continues to echo with sorrow.
