The strength of the diabolical lies in the power of deception. In subtle ways, it destroys the bonds of friendship and family, distorts the beliefs of the faithful, and takes possession of the living.

Sarah Miller perceives that something supernatural lurks inside her new home, terrorizing her children, and causing her to doubt her own senses. Her fiancé's skeptical response provokes frustration, putting the entire house on-edge, as a sinister presence struggles to take control of the family.

Sarah seeks help from a lay religious demonologist, who recognizes the symptoms of a diabolical infestation, and prepares an ancient ritual to expel the evil, before it destroys the family from within.