In 1981, renowned demonologists Ed Warren and Lorraine Warren find themselves deeply involved in a harrowing case involving the exorcism of 8-year-old David Glatzel, which is attended by his family members, including Debbie Glatzel, her boyfriend Arne Johnson, and Father Gordon (Steve Coulter). During the intense ritual in the town of Brookfield, Connecticut, Arne boldly invites the demon to take over his own body instead of David's, a moment that leads to dire consequences.
As Ed witnesses the demon transferring from David to Arne during a critical moment while he suffers a heart attack, he is left comatose and hospitalized. Once he awakens, Ed reveals to Lorraine his unsettling realization about Arne’s possession. In a desperate move to prevent impending doom, Lorraine alerts the police to rush to a kennel, foreseeing that a tragedy is about to occur. Shortly after, the couple returns to their apartment above the kennel where Debbie works, but their lives take a dark turn when Arne, under the influence of demonic forces, brutally stabs his landlord Bruno Sauls 22 times.
With the Warrens' assistance, Arne's case becomes unprecedented as the first American murder trial to assert demonic possession as a legal defense, igniting an investigation into the original possession of David. The Warrens uncover a sinister satanic curse linked to a witch’s totem, leading them to Kastner, a former priest familiar with the Disciples of the Ram cult. Kastner discloses that a malevolent occultist had deliberately left the totem behind, casting a curse upon the Glatzel family that ultimately led to David’s torment.
The investigation takes them to Danvers, Massachusetts, where the tragic death of Katie Lincoln is revealed, also having been stabbed 22 times. A totem found at Jessica's home—a friend of Katie's who has mysteriously gone missing—links to the occultist. Through a harrowing vision, Lorraine witnesses the tragic sequence of events where Jessica, influenced by the same demonic possession, stabs Katie before taking her own life off a cliff. This revelation helps the detectives locate Jessica's body.
The search intensifies when Lorraine seeks the occultist at the funeral home, connecting directly with the corpse to unveil its secrets. In a chilling vision, she travels through shadowy tunnels to thwart the occultist’s intentions, just as Ed finds himself on the brink of being possessed himself but is saved in the nick of time by Drew.
Back at their home, they uncover the same cursed totem concealed among black roses. A book detailing Stregherian witchcraft provides crucial insights, revealing that to lift the curse, the occultist’s altar must be annihilated. Their inquiries lead to the assumption that the occultist is operating near Fairfield University, where Katie had attended.
Desperate for aid, Lorraine returns to Kastner, who regretfully shares his dark past involving his daughter Isla, who grew fascinated with the occult and ultimately became the very occultist they seek. Tragically, Kastner loses his life in the ensuing confrontation, but not before guiding Lorraine toward the altar’s location.
In a gripping climax, Ed breaches the tunnels with determination, yet momentarily succumbs to possession. Just as he plans to harm Lorraine, she courageously reminds him of their love and first meeting, allowing him to regain control. With newfound resolve, Ed destroys the altar, saving all involved from the demonic threat. In a twist of fate, the occultist meets her demise at the hands of the very demon she summoned when her curse remains incomplete.
In the aftermath, Ed adds the remnants of the altar to their collection of artifacts, placing it alongside the Valak painting and the Annabelle doll for safekeeping. Meanwhile, Arne receives a manslaughter conviction but only serves five years, ultimately marrying Debbie during his imprisonment. To commemorate their bond, Ed presents Lorraine with a replica of the gazebo where they first shared a kiss, symbolizing their enduring love amidst the darkness they've faced.
