True-crime writer Ellison Oswalt moves his family into a new home, unaware that the house is the site of a horrific murder of a family. Intent on writing his next bestseller, he discovers a box of mysterious and disturbing home movies in the attic that plunge him into a nightmarish investigation. Each film reveals a family being brutally murdered in various ways, with an unseen figure lurking in the footage. As Ellison delves deeper, he becomes obsessed with uncovering the truth behind the killings, hoping to write a hit book and revive his floundering career.

While researching the murders, Ellison notices a ghastly recurring figure in the films—a pagan deity named Bughuul, who may be more than just a myth. With a strange set of symbols appearing in the films, Ellison consults with a local deputy and a supernatural expert to decode the meaning behind them. Meanwhile, Ellison's wife Tracy and their two children begin to sense something amiss in the house. As mysterious occurrences increase and the family's safety is compromised, Ellison must grapple with the reality that his ambition may have exposed them to an ancient and malignant evil.