Mia Hu, Michelle Ang, Gavin, Brett Gentile, and Luis, Jeremy DeCarlos, are a team of students who want to create a documentary about Deborah Logan, Jill Larson, an elderly woman living with Alzheimer's and cared for by her daughter, Sarah Logan, Anne Ramsay. As the film crew shadows Deborah’s daily routine, her behavior grows increasingly strange, and her personal physician argues that such actions can be explained by the progression of a harsh form of the disease. Yet cameraman Luis begins to suspect there is something more than medical symptoms at play, a suspicion that deepens when the team records Deborah speaking in French about sacrifices and snakes.
They notice a troubling pattern: the switchboard line for 337 incessantly rings, a line that belonged to local physician Desjardins, Kevin A. Campbell, who disappeared after a chain of cannibalistic ritual murders targeting four young girls. Deborah’s conduct becomes so dangerous that she is hospitalized for her own safety, and Gavin withdraws from the project in dismay. Undeterred, the trio—Mia, Gavin, and Luis—press on and uncover a chilling theory: Desjardins was attempting to revive an ancient Monacan ritual that would grant immortality, but it required the deaths of five young girls who recently began menstruating. The team wonders if Deborah has become a vessel for Desjardsins’ malevolent influence.
Soon, they learn that Deborah had previously tried to abduct Cara Minetti, a young cancer patient portrayed by Julianne Taylor. At the hospital, Harris Sredl, Ryan Cutrona, visits Deborah and she pleads with him to end her life. He tries to help, but is knocked unconscious by the entity possessing her. Through Harris, Sarah discovers that years earlier Deborah learned Desjardins’ plan to use Sarah as his fifth victim, which pushed Deborah to kill the doctor before the ritual could reach its goal. She then buried the doctor’s body in the yard. The united group finds the remains and attempts to burn them, but Desjardins’ spirit intervenes and thwarts their efforts.
Desperate, Deborah abducts Cara again and takes her to the location where Desjardins had slain his previous victims. As Deborah heads into the mines, Sheriff Linda Tweed, Tonya Bludsworth, trails after her and is killed in the ensuing chaos. Sarah and Mia locate Deborah, who is in the act of a horrifying snake-like movement aimed at Cara’s head; they manage to burn Desjardins’ corpse, and Deborah is briefly restored to awareness. In a quiet, hopeful moment, Sarah and the now-reconciled Deborah reunite, but the sense of danger lingers.
The film closes with a montage of news footage reporting that Deborah was deemed unfit to stand trial and that her health deteriorated rapidly in the months after the incident. Cara, who has survived cancer, celebrates her 10th birthday, yet a final image cuts to Cara facing the camera with a cold, unsettling smile—an ominous hint that Desjardins’ presence may now be haunting her.
