The film opens with a flashback to Judy Warren's birth, where an ominous encounter involving an antique mirror triggers Lorraine's early, perilous labor. In the delivery room, Lorraine glimpses a shadowy entity watching from the ceiling before giving birth to a healthy baby girl. Decades later in 1986, Ed and Lorraine Warren are in semi-retirement, focusing on academic university lectures while Ed manages his fragile heart condition. Meanwhile, an adult Judy announces her engagement to Tony Spera, a former police officer who devoted his life to Judy after miraculously surviving a close-range shooting.

In West Pittston, Pennsylvania, the Smurl family moves into a duplex shared with Jack's elderly parents. At young Heather's birthday, her grandfather gifts her an antique mirror adorned with hand-carved cherubs, unaware it is the same cursed object the Warrens encountered years prior. Supernatural torment begins almost immediately: phone cords are violently pulled into empty closets, mechanical toys move on their own, and an apparition of a snarling old hag appears to the children. When the sisters attempt to discard the mirror, one of the girls violently vomits blood and broken glass shards into the street.

As the physical attacks escalate to levitation, phantom assaults, and an apparition of an axe-wielding maniac, Jack Smurl takes their story to the public press. Desperate for help, Father Gordon implores the reluctant Warrens to investigate. Although Ed and Lorraine originally intend to decline due to Ed's failing health, Judy experiences terrifying clairvoyant visions of the hag and insists on intervening to save the children.

The Warrens, Judy, and Tony arrive at the Smurl residence to conduct an investigation. Lorraine discovers the mirror acts as an unholy conduit binding three distinct spirits: an old woman, a violent woodsman, and a young girl, all controlled by an ancient demon seeking to harvest human souls. During an intense vigil, the demon attempts to force one of the possessed girls to hang herself in the attic, causing the floorboards to collapse and plunging the rescuers into the level below.

Realizing the demon intends to exploit Ed's weak heart to shatter Lorraine's will, the family stages a high-stakes spiritual confrontation inside the Smurl house. The demon manipulates reflections, shattering the glass and manifesting physically to drag Judy into the void. Ed, armed with his deep faith, steps in to shield his daughter, while Lorraine and Judy recite the holy rites in unison. Together, they break the demon's hold over the trapped human souls, shattering the curse and expelling the entity permanently back to the abyss.

With the Smurl home cleansed and the family free from torment, Ed and Lorraine formally declare their field investigations complete, passing their blessing to Judy and Tony as they prepare to build a peaceful future together.

The final sequence returns to the Warrens' Connecticut home. Ed and Lorraine walk through their famous occult artifact room one last time, reflecting on decades spent standing shoulder to shoulder against the dark. Ed turns off the overhead lights, leaving the museum silent and secure behind locked doors. A closing title card pays tribute to the real-life Ed and Lorraine Warren, celebrating their enduring marriage, lifelong faith, and the countless families they comforted over a lifetime of service.