Dr. Mara Voss, a young medical anthropologist recovering from a scandal, takes a short-term position documenting a longitudinal cognitive study at Saint Elara Care Home. The study's lead subject is 72-year-old Deborah Langford, a sharp, voluble former rare-books librarian now slipping into apparent Alzheimer’s. Mara hopes this clean, well-funded project will rebuild her reputation — footage, interviews, and behavioral logs intended for a peer-reviewed paper and a charity documentary.

As the film progresses, the crew discovers that Deborah has become possessed by some kind of supernatural entity. The entity, which seems to have a life of its own, begins to manipulate Deborah, causing her to perform heinous acts and putting the entire crew in grave danger.