Kristen Bouchard, a forensic psychologist, speaks with Orson Leroux, a man accused of multiple murders and rapes, to determine his sanity to stand trials for his alleged crimes.
Leroux claims to have blacked out during the attacks, but Kristen testifies for the Queens District Attorney's Office that Leroux's account of events can't be trusted, as an administered psychological test placed him in the deceitful range.
However, Leroux's lawyer counters that Leroux didn't commit the murders and rapes of his own volition, as, according to an affidavit signed by Leland Townsend, another forensic psychologist, Leroux is possessed by a demon named Roy.
To disprove the lawyer's claim, Kristen re-interviews Leroux, who swears he isn't possessed by a demon. However, as Kristen quotes scripture at Leroux, "Roy" supposedly makes an appearance as Leroux jumps on the table and lunges at Kristen. Kristen is thrown to the floor, and "Roy" chokes her and starts speaking Latin.
Following her interview with Leroux, Kristen, who is unsure of the truth, tells the prosecution she can no longer confirm her earlier testimony without further assessment. When she is told by the Queens District Attorney's Office that isn't possible, she quits.
Priest-in-training David Acosta, who was hired by Leroux's wife to determine whether her husband is possessed later, shows up on her doorstep and offers her a job working with the Church to investigate its backlog of unexplained phenomena.
Acosta says insanity and possession look very similar, and he needs someone like Kristen to help distinguish between two. While Kristen doesn't believe in the supernatural, she accepts Acosta's offer because she needs the money to repay her student loans.
The following day, Kristen, Acosta, and contractor Ben Shakir, go to Leroux's house to investigate. Despite the wife's audio recording which could indicate the presence of a demon, Kristen is still just as skeptical about ghosts and demons.
When she falls asleep that night, she gets an eery visit from a demon named George who insists he is real and asks her if she's wearing any underwear. Kristen then wakes up screaming and is comforted by the knowledge that it was just a nightmare.
Kristen later tells her therapist about her nightmare, and the therapist prescribes her medication to help her sleep.
Kristen, Acosta, and Ben return to Leroux's house, and Ben determines the hissing sound in the audio recording was caused by a faulty dishwater. Ben tells Kristen that he, like her, is a nonbeliever, but Acosta, who was a thrill seeker before joining the priesthood, is a true believer.
Kristen and Acosta visit Leroux together this time, who continues to claim he doesn't remember the killings or who Roy is.
That night, Kristen is once again visited by George, who asks her various questions about her attraction to Acosta. However, she realizes it's just a nightmare as she cannot read the sign she posted on her ceiling.
When Kristen and Acosta go to visit Leroux again, Kristen is freaked out that Leroux, or "Roy" knows about George. Kristen doesn't understand how Leroux could have known until she goes to her therapist's office and finds out the notes from their sessions were stolen.
Kristen then learns that Townsend stole the notes and slipped them to Leroux, though she isn't sure why. They get into a heated conversation outside the courtroom after Townsend testifies, and he warns Kristen that she should leave this sort of work to the professionals.
Acosta then shows up and punches Townsend when the forensic psychologist mentions Acosta's friend named Julia. Acosta then goes to a bar, and Kristen joins him.
Acosta tells her Townsend is a "connector," or an individual who's real pursuit in life is evil and encouraging others to do evil. He says Kristen doesn't have to believe in the supernatural to know that there are people out there who do bad things and encourage others to do the same.
Acosta and Kristen then realize Townsend may have connected with Leroux online and encouraged the suspected serial killer to commit the murders. After some more investigating, they learn Leroux used his wife's email account and met Townsend online. Leroux was then coached by Townsend on how to appear demonically possessed so he could get away with the murders and rapes.
Kristen then gives the evidence to the prosecutor and decides to officially join the team.