The film begins with an intense exorcism involving two priests attempting to liberate Hannah Grace (Kirby Johnson) from a malevolent force that has taken over her body. Her father, Grainger (Louis Herthum), watches in despair, urging his daughter to resist the demon that has engulfed her. Just as the situation escalates, Hannah's body becomes a weapon for the dark entity, launching the priests into chaos—one gets thrown against the wall, resulting in a horrific impalement, while the other finds himself choking. In a desperate move, Grainger smothers Hannah, hoping to save her from the torment.
Fast forward three months, Megan Reed (Shay Mitchell) arrives in Boston, ready for her new position at Boston Metro Hospital after being enlisted by her friend and AA sponsor, Lisa (Stana Katic). During her interview with Dr. Henry Lewis (James A. Watson Jr.), she reassures him that she can handle the graveyard shift in the morgue, despite the unsettling atmosphere and the lingering odor of death.
As her first night on the job begins, she encounters security guards, Ernie (Jacob Ming-Trent) and Dave (Maximillian McNamara), providing a momentarily comforting routine. However, as she prepares for her duties, a haunting memory surfaces from her past—one where her hesitation in the line of duty led to the death of her partner, resulting in a painful spiral into substance abuse and rehab.
The subsequent day, Megan's ex-boyfriend Andrew (Grey Damon) visits to collect his belongings. Their conversation hints at lingering tensions and unresolved feelings, culminating in a confrontation that further complicates her mental state.
Later that night, an unsettling encounter unfolds when Megan confronts a hooded figure—Grainger—demanding entry into the morgue. Unnerved, she denies him, but distraction soon follows as Randy (Nick Thune) delivers a mutilated body to her. To her horror, she realizes the body belongs to Hannah, leading her to experience bizarre and horrifying hallucinations, including flies swarming around the body.
As the nights progress, paranormal instances escalate, such as the chilling disappearance of Hannah's body and Grainger’s violent outburst that leaves Megan shaken. Despite her attempts to rationalize the circumstances—the breathing corpse or Hannah's eerie appearances—Megan’s grip on reality continues to falter.
With mounting tension and a rising body count, Megan seeks to understand the connection between her past trauma and the spectral presence of Hannah. Each character falls prey to the cursed entity: Lisa meets a gruesome fate on the rooftop, while Dave suffers a horrific death in the morgue. The revelation that Hannah's body regenerates with each death instills urgency in Megan and Andrew as they devise a plan to destroy it once and for all.
In a final showdown, after enduring the relentless terror brought on by Hannah, and witnessing Ernie’s brutal murder, Megan courageously confronts her fears by putting an end to Hannah's reign of horror. With a shot fired in desperation, she manages to push Hannah into the incinerator, despite her pleas for mercy.
In the aftermath, as Megan starts to reclaim her life, a simple moment—a fly on the mirror—draws her back to the unsettling reality she faced, hinting that perhaps the past is never truly behind us.
