The Exorcist III (1990)

The film begins with the point of view of someone wandering through the streets of Georgetown, a voice informing us "I have dreams... of a rose... and of falling down a long flight of stairs." The point of view shows a warning of evil about to arrive later that night at a church. Demonic growls are heard. Leaves and other street trash suddenly come flying into the church as a crucifix comes to life. It then cuts to Lieutenant William F. Kinderman at a crime scene, where a 12-year-old boy named Thomas Kintry has been murdered.

Kinderman takes his friend, a priest named Father Dyer, out to see their mutually favorite film It's a Wonderful Life. Kinderman later relates the gruesome details of the murder of the young boy he was investigating that morning, including his crucifixion. Another murder soon takes place - a priest is found decapitated in a church. Dyer is shortly hospitalized—and found murdered the next day - with the words "IT'S A WONDERFULL LIFE", written on a wall in Dyer's blood.

The fingerprints at the crime scenes do not match, indicating a different person was responsible for each. Kinderman tells hospital staff the reason for his unease: seventeen years ago the vicious serial killer James "The Gemini" Venamun, was executed; with every victim he cut off the right index finger and carved the Zodiac sign of Gemini into the palm of their left hand. Kinderman noticed the hands of the three new victims and verified that the Gemini's sign has been there. The Gemini Killer also always used an extra "L" in his notes sent to the media, such as "usefull" or "carefull". Furthermore, to filter out false confessions, the original Gemini Killer's true mutilations were kept a secret by the Richmond police's homicide department; the newspapers were made to wrongfully report that the left middle finger was severed and that the Gemini sign was carved on the back of the victim.

Kinderman visits the head of the psychiatric ward, Dr. Temple, who relates the history of a man in Cell 11, that he was found wandering aimlessly seventeen years ago with amnesia. The man was locked up, catatonic until recently when he became violent and claimed to be the Gemini Killer. Kinderman sees that the patient resembles his dead friend Father Damien Karras. The patient expresses ignorance of Father Karras, but boasts of killing Father Dyer.

The next morning, a nurse and Dr. Temple are found dead. Kinderman returns to see the patient in Cell 11, who claims that after his execution his soul entered Karras's dying body. The Gemini's spiritual "master", who had possessed the girl Regan MacNeil, was furious at being pushed out of the child's body and is exacting its revenge by putting the soul of the Gemini Killer into the body of Father Karras. Each evening, the soul of the Gemini leaves the body of Karras and possesses the elderly people with senile dementia elsewhere in the hospital and uses them to commit the murders. The Gemini Killer forced Dr. Temple to bring Kinderman to him or he would suffer in unspeakable ways — Temple couldn't take the pressure, and he committed suicide.

The Gemini possesses an old woman, who makes a failed attempt to murder Kinderman's daughter. The possessed patient attacks Kinderman, but the attack abruptly ends when a priest, Father Paul Morning (Nicol Williamson), enters the corridor leading to cell 11 and attempts an exorcism on the patient. The Gemini's "patron" intervenes, taking over the patient's body, and the priest is all but slain. Kinderman arrives in time and attempts to euthanize Karras after finding the body of the priest but is hurled into the wall by the possessed Karras. Father Morning manages to briefly regain consciousness and tells Karras, "Damien, fight him." Karras regains his free will briefly and cries to Kinderman, "Bill, now! Shoot now! Kill me now!" Kinderman fires his revolver several times, hitting Karras in the chest, fatally wounding him. The Gemini is now gone...and Karras is finally free. With weak breaths, he says "We won, Bill. Now free me." Kinderman puts his revolver against Karras' head—and fires.

The film ends with Kinderman standing over Karras' grave.

The Exorcist (1973)

In 1973, Lankester Merrin is a veteran Catholic priest and exorcist who is shown to be on an archaeological dig in the ancient city of Hatra in Iraq. There he finds an amulet that resembles the statue of Pazuzu, a demon of ancient origins with whose history Merrin is familiar.

In modern-day Georgetown, actress Chris MacNeil is living on location with her 12-year-old daughter Regan, where Chris has just wrapped the final scene of a film about student activism directed by her friend and associate Burke Dennings. After playing with a Ouija board and contacting a supposedly imaginary friend whom she calls Captain Howdy, Regan begins acting strangely, including making mysterious noises, stealing, constantly using obscene language, and exhibiting abnormal strength. Chris hosts a party, only for Regan to come downstairs unannounced, telling one of the guests, who is an astronaut, "You're gonna die up there", and then urinating on the floor. Later, Regan's bed begins to shake violently, adding further to her mother's horror. Chris consults a number of physicians, but Dr. Klein and his associates find nothing physiologically wrong with her daughter, despite Regan undergoing a battery of diagnostic tests.

One night when Chris is out, Burke Dennings is babysitting Regan, only for Chris to come home to hear he has died falling out the window. Although this is assumed to have been an accident, given Burke's history of heavy drinking, his death is investigated by Lieutenant William Kinderman, who interviews Chris, as well as priest and psychiatrist Father Damien Karras, who has recently been emotionally shaken after the death of his frail mother.

The doctors, thinking that Regan's aberrations are mostly psychiatric in origin, recommend an exorcism to be performed. Chris arranges a meeting with Karras. After recording Regan speaking backwards and witnessing the apparent effect of the scarification of her flesh with the words "Help Me" on her stomach, Karras is convinced Regan is possessed. Believing her soul is in danger, he decides to perform an exorcism. The experienced Merrin is selected for performing the actual exorcism with Karras present to assist.

Both priests witness Regan perform a series of bizarre, vulgar acts, and confine her to her bedroom. They attempt to exorcise the demon, but a stubborn Pazuzu toys with them, especially Karras. Karras shows weakness, and is dismissed by Merrin, who attempts the exorcism alone. Karras enters the room and discovers Merrin has died of a heart attack. After failing to revive Merrin, the enraged Karras confronts the mocking, laughing spirit of Pazuzu, tackling the demon to the ground. At Karras's furious demand, Pazuzu then possesses Karras, leaving Regan's body. In a moment of self-sacrifice, the priest throws himself out of the window before allowing Pazuzu to compel him to harm Regan, and is himself mortally injured. Father Dyer, an old friend of Karras, happens upon the scene and administers the last rites to his friend.

A few days later, Regan, who is now back to her normal self, prepares to leave for Los Angeles with her mother. Although Regan has no apparent recollection of her possession, she gives Father Dyer a kiss on the cheek. Kinderman, who narrowly misses their departure, befriends Father Dyer as he investigates Karras's death.