The movie Nefarious follows the story of Dr. Richard Bell, a seasoned psychiatrist who has evaluated several criminals over the years. He is assigned to evaluate Simon Haines, a notorious serial killer who has been sentenced to death. Dr. Bell attends Haines' evaluation with the expectation of carrying out a routine assessment. As the evaluation progresses, however, Haines makes a shocking claim. He insists that he is not human, but a demon that has possessed the body of a human being. He claims to have lived for centuries, committing heinous crimes, and evading justice throughout history.
Dr. Bell initially dismisses Haines' claims as delusional, but he soon realizes that there may be more to it than he thought. As Bell delves deeper into Haines' mind, he begins to uncover shocking truths about his patient's past. Haines' claims of being a demon appear to be supported by a wealth of historical and mythological evidence.
Meanwhile, Haines makes some chilling predictions about the future. He claims that before the end of the evaluation, three murders will take place, and Dr. Bell will be the one to commit them. Bell is understandably disturbed by Haines' words, but he dismisses them as the ramblings of a deranged criminal.
As the evaluation continues, however, Dr. Bell begins to experience terrifying visions, implicating him of the very crimes that Haines predicted. With this doubt undermining Dr. Bell's mental state, he loses grip of his sanity as he struggles to uncover the truth of his patient's claims and the impending reality they may bring. The movie Nefarious escalates into a violent and explosive finale as Simon Haines' master plan unfolds and Dr. Bell plunges into the abyss of his own psyche.