One night, Chris finds the house empty except for a sleeping Regan. Dennings is found dead at the foot of an outdoor staircase beneath Regan's window. Homicide detective William Kinderman questions Karras, confiding that Dennings' body was found with its head turned backwards. Regan's condition worsens, and her body becomes covered with sores. A doctor mentions exorcism as a remote option, suggesting a possible psychological benefit. Kinderman visits Chris, explaining that the only plausible explanation for Dennings' death is that he was pushed from Regan's window. As Kinderman leaves, the possessed Regan stabs her genitals with a crucifix. To Chris' horror, the possessed Regan turns her head backwards and speaks in Dennings' voice. The possessed Regan is confined to her bedroom. Chris seeks out Karras, who visits Regan. Over two meetings, the possessed Regan claims to be the Devil himself, projectile vomits into Karras' face, speaks in tongues, and reacts violently when tap water is sprinkled on her, which Karras had claimed was holy water—a point against genuine possession. The demon says it will remain in Regan until she is dead. Desperate, Chris confides that the possessed Regan killed Dennings. At night, Chris' assistant calls Karras to the house. They witness the words "help me" materialize on Regan's skin. Still ambivalent, Karras nevertheless concludes that an exorcism is warranted. His superior grants permission on the condition that an experienced priest lead the ritual while Karras assists. Merrin, having performed an exorcism before, is summoned. Merrin arrives at the house, warning Karras that the demon uses a psychological attack. As the priests read from the Roman Ritual, the demon curses them. It focuses on Karras, verbally attacking his loss of faith and guilt over the circumstances of his mother's death. The priests rest momentarily and Merrin, shaking, takes nitroglycerin. Karras enters the bedroom where the demon appears as his mother. Showing weakness, Karras exclaims that the demon is not his mother. Merrin excuses Karras and continues the exorcism by himself. Karras assures Chris that Regan will not die and re-enters the room, finding Merrin dead. Enraged, Karras beats the possessed Regan and demands that the demon take him instead. The demon rips a medallion of Saint Joseph from Karras' neck and begins to possess him, freeing Regan. Karras hurls himself out the window, tumbling down the stairs outside. Chris and Kinderman enter the room. Chris embraces the healed Regan, and Kinderman