Jesuit priest Joseph Dyer visits the outdoor stairway in the Georgetown neighborhood of Washington, D.C., where his friend, Father Damien Karras, tumbled to his death fifteen years earlier. Meanwhile, Police Lieutenant William “Bill” Kinderman looks at a photograph of himself with Father Damien. That night, as wind blows open the doors of nearby Holy Trinity Church, Father Dyer dreams of falling down the steps, while Kinderman walks a deserted street and receives a rose from a young black man. The next day, Kinderman is called to the Potomac River to investigate the mutilation murder of a twelve-year-old African-American boy, Thomas Kintry. Recognizing the modus operandi of James Venamun, known as the “Gemini Killer,” he asks his detectives to get the serial killer’s file. They remind him that Venamun was executed fifteen years earlier. Later, Kinderman and Father Dyer attend a screening of Dyer’s favorite movie, It’s a Wonderful Life, and afterward have a discussion in which Kinderman discusses how Kintry, the latest victim, was ritually decapitated and laid out with a head from a statue of Jesus, painted in blackface. Elsewhere, as Father Kanavan takes confession at Holy Trinity Church, an old woman tells him how she once butchered a child. Later, Kinderman arrives to investigate Father Kanavan’s murder. The priest’s head and right index finger are gone. Later, Kinderman goes to Georgetown General Hospital to talk with Father Dyer, who has admitted himself for medical tests. In a hallway near the elevators, Kinderman fails to notice that a statue of Jesus in a wall niche is missing its head. Sergeant Adkins reports to Kinderman that both Kintry and Father Kanavan were drugged with a paralyzing agent before being mutilated, but despite the similarities in the crimes, fingerprints at the two scenes do not match. That night, Kinderman dreams of walking through a purgatory-like waiting room, serenaded by an orchestra of angels. He sees Kintry, then Father Dyer, and asks the priest if they might be having the same dream. Dyer replies that the detective is not dreaming. The telephone awakens Kinderman, and he is told that Father Dyer has been murdered. Arriving at Georgetown General Hospital, he learns the priest’s blood has been drained and transferred to rows of specimen jars beside the bed. Scrawled in blood on the wall are the words “It’s a Wonderfull Life.” Kinderman orders the hospital locked down and searched. Nurse Allerton tells the lieutenant that an elderly, “quasi-catatonic patient,” Mrs. Clelia, was lying in the hallway outside Dyer’s room that morning. Staff psychiatrist Dr. Temple introduces Kinderman to her, but Mrs. Clelia remembers nothing. Dr. Temple ushers Kinderman into the “disturbed ward” through a security door. Hearing someone call his name, Kinderman looks into a dark cell and sees a man shackled in a straitjacket who looks like Father Damien. Dr. Temple identifies him as “Patient X.” Before Kinderman can investigate further, Dr. Freedman, Georgetown General’s administrator, calls him to his office to demand that he remove his policemen from the hospital. Kinderman explains that Father Dyer’s murder, as well as two previous ones, bear the marks of the Gemini Killer: The right index fingers were removed, and a Zodiac sign was carved into the left palms. Also, whoever wrote the blood note on Father Dyer’s wall spelled “wonderful” with two “l”s, another trait of the Gemini Killer. They go to an operating room, where Kinderman sees a set of spring-activated surgical shears capable of decapitating a human. Kinderman visits the Jesuit president of Georgetown University to discuss what the victims had in common. Father Dyer introduced Father Damien to the MacNeil family, whose daughter Regan was given exorcism rites fifteen years earlier by Father Damien and another priest. Father Kanavan authorized the exorcism, and Kintry’s mother, who analyzed the tapes of the demon speaking through Regan MacNeil, determined that its language was English spoken backwards. The president reveals that Father Paul Morning, who lives on campus, is the only priest in the diocese who has performed an exorcism. Nearby, Father Morning witnesses strange things in his room. A bird he rescued suddenly dies, a crucifix falls off the wall and bleeds, and wind blows open the windows. Later, Kinderman learns that Mrs. Clelia’s fingerprints are on the jars containing Father Dyer’s blood. He asks why she went into the priest’s room, but the old woman has no memory of doing it. When Kinderman asks Dr. Temple about “Patient X,” the psychiatrist explains that he was an amnesia victim without identification who was brought to the ward fifteen years earlier. Recently the man became violent, which is why he is now shackled. When Dr. Temple says Patient X imagines himself to be the Gemini Killer, Kinderman tells his detectives to get the late Father Damien’s dental records, because he believes the priest is Patient X. Taken to the cell, Kinderman finds Patient X anxious to talk. The prisoner gives the detective details of the Gemini Killer’s murders that only the perpetrator would know, and confesses he recently killed the priests and the Kintry boy “for a friend.” He demands that Kinderman alert the press that the Gemini Killer is back, or else “I will punish you,” then mumbles “goodbye, Amy,” and falls asleep. Kinderman learns from Nurse Allerton that the locks on Patient X’s shackles keep breaking. She mentions that she once heard him say, “save your servant,” in a normal voice. Kinderman hurries to Georgetown University library and finds the words—“save your servant who trusts in you, my Lord”—in a passage from the “Rite for Exorcism.” Later, in the disturbed ward, nurse Amy Keating is decapitated and her internal organs replaced with Catholic rosaries. Dr. Temple is also found dead from a drug overdose. Patient X asks Kinderman if he “got my message.” He explains that his soul was stranded in a void after his state execution, but when Father Damien exorcised a demon from Regan MacNeil, it leaped into Father Damien’s body and brought the Gemini Killer’s spirit with him. Now, the demon enjoys mentally torturing Father Damien, who cannot stop the evil perpetrated by his temporal body. Patient X tells Kinderman he also controlled Dr. Temple, which is why the psychiatrist brought the lieutenant to his cell in the first place. Asked how he got access to his victims, Patient X coyly admits having “old friends” who do it for him. Again, Patient X drifts off, and for a moment, Father Damien calls out for “Bill” to help him. Returning to the main ward, Kinderman realizes the “old friends” must be the senile and semi-catatonic patients. In a nearby room, an old woman murders a young nurse, dresses in her uniform, and escapes by crawling along the ceiling above Kinderman’s head. Seeing blood coming from under a door, the lieutenant finds the dead nurse nude, and fears that Patient X has sent a patient, now dressed as a nurse, after his family. He tries to telephone his wife, Mary, but the line is busy. However, Mary receives a telephone call from someone sounding like her husband, announcing that a nurse is coming to the house. A policeman drives Kinderman home with the siren screaming, and when he arrives, the nurse sits in the kitchen with Mary and daughter Julie. The elderly nurse murmurs “help me,” then reaches into her bag and pulls out a large pair of shears. She tries to cut off Julie’s head, but Mary pulls her daughter away in time. The nurse throws the lieutenant against the wall and starts to strangle him, but suddenly loses strength and falls to the floor. At that moment, Father Morning enters Georgetown General’s disturbed ward, and Patient X knows why he has come. The priest, carrying a Bible, a crucifix, and holy water, performs the rite of exorcism, but the demon’s power sets the Bible on fire and presses Father Morning against the ceiling, cracking open his skull. The priest lies on the floor when the police detective returns to the cell. Kinderman realizes he must kill Father Damien’s body in order to end his torment and trap the demon, but Patient X throws him against the wall. Despite his agnosticism, Kinderman shouts that he believes in the devil, because he believes in corruption and evil. Lightning bolts open a vision of hell in the floor of the cell, and murder victims rise up. With his last breath, Father Morning holds up his crucifix long enough to distract the demon and break its hold on the lieutenant. Kinderman fires a bullet into Father Damien’s body.