Necromaniac depicts the harrowing descent of a thirty-three-year-old woman who fears she is losing her grip on sanity. Or is she being possessed? Managing a crematorium, where she also lives as a virtual recluse harboring a dark secret and taking refuge from the world, Libby Genet fears she is going mad when she feels an uncontrollable urge to kill. When her psychiatrist can't cure her, Libby comes to the conclusion that she might be in the process of being possessed, since she hears a voice in her head which keeps commanding her to commit murder. Fighting to take control of her life, she hires an eighteen-year-old Hollywood psychic who bills himself as "Brady the Believer." A matricide recently released from an institution for the criminally insane, he tells Libby they must find out which unsettled spirit is trying to possess her before he can exorcise it. The spirit could be from any of the hundreds of corpses she has cremated, including several from a local prison. Is it the spirit of a serial killer that Libby cremated recently or could it be that of a Mafia enforcer she cremated years ago? Or could it be that of someone much more sinister?

Libby and Brady race the clock to find the answer before the demon completes its possession of Libby, turning her into a serial killer who can never be exorcised.