As the Montelli family moves into their new residence at 112 Ocean Avenue in Amityville, New York, an unsettling atmosphere settles over them. Comprising Anthony and his wife Dolores, along with their children Sonny, Patricia, Mark, and Jan, they remain blissfully unaware of the sinister force lurking within their new home. Strange events begin to unfold: doors creak open at night, and a chilling message scrawls itself across the wall of Jan's room, bearing the unmistakable mark of darkness.
Tensions mount as Anthony's temper flares. He wrongly blames his children for the eerie happenings while Dolores struggles to maintain harmony within the household. The arrival of Father Frank Adamsky, who comes to bless their home, escalates the situation further. A confrontation between him and Anthony, filled with heated arguments and resulting in a physical altercation, leaves the family fractured. The usually steadfast priest is left in shock, departing with a torn Bible and shaken faith.
As time goes by, the Montelli family's foundation crumbles. Anthony's demeanor becomes increasingly unstable, shifting from a strict father to a violent force, compelling Dolores to tread carefully to ensure the safety of their younger children. In the midst of this turmoil, Sonny's growing curiosity leads him to investigate a disturbing sound in the basement. This fateful decision culminates in his possession by the very evil that has been haunting them all along.
As the once-close family spirals into chaos, Patricia becomes enmeshed in a grotesque game of make-believe with Sonny, who is now under demonic influence. Their disturbing actions are just one manifestation of the dark powers gripping their home, leaving the surviving family members questioning their freedom from this malevolent presence.
As the demonic force further ensnares Sonny, his facial expressions morph into unsettling, sinister forms. Despite his efforts to protect his loved ones from the burgeoning evil within, he finds himself entirely dominated. On his birthday, Sonny isolates himself, but Patricia feels compelled to check on him. Her hollow expressions of concern fall on deaf ears as Sonny's demonic episodes push him to the brink of violence, culminating in a brutal confrontation that sends Patricia fleeing in tears. She finds herself unable to connect with the oblivious Adamsky, who dismisses her worries.
As darkness envelops the night, the evil within Sonny commands him to unleash a wave of savagery upon his family. He complies, resulting in the tragic deaths of Jan, Mark, and ultimately, Patricia, after a relentless pursuit. The following day, authorities arrive to collect the bodies and arrest Sonny. The horrific truth begins to surface, but Sonny's memories of the grisly events remain shrouded in confusion, leading Adamsky to conclude that he's dealing with a case of possession.
With the Catholic Church unwilling to authorize an exorcism, Adamsky takes matters into his own hands. He springs Sonny from police custody and leads him to the church. However, when faced with sacred symbols at the doors, Sonny's evil resurfaces, resulting in an escape and a subsequent attack. Refusing to give up, Adamsky follows him back home and conducts a makeshift exorcism, liberating Sonny from the clutches of darkness.
As authorities close in, Adamsky pleads with Father Tom to take Sonny into protective care before the police re-arrest him, leaving both their fates unknown. The foreboding specter of the demon hints at having transferred its dark influence onto the unsuspecting priest. In the aftermath, the once-ominous house is put on the market, setting the stage for the next dark chapter in its history, soon to be inhabited by the Lutz family.
