The film opens with a haunting perspective as someone strolls through the streets of Georgetown, accompanied by a disembodied voice echoing, "I have dreams... of a rose... and of falling down a long flight of stairs." This unsettling viewpoint reveals a sinister presence approaching a church later that night. As chaos ensues, ominous growls pierce the air while debris flings itself towards the church, culminating in a crucifix manifesting a life of its own.
We are then introduced to Lieutenant William F. Kinderman, portrayed by George C. Scott, at a gruesome crime scene where the brutal murder of a 12-year-old boy named Thomas Kintry has taken place. In an effort to seek solace, Kinderman invites his friend, Father Dyer, played by Ed Flanders, to watch their cherished film, It's a Wonderful Life. Over popcorn, Kinderman darkly recounts the murder details, including the shocking manner in which the boy was killed, leaving behind a grim atmosphere.
The tension escalates when another murder rocks the community; a priest is gruesomely discovered decapitated within the very walls of a church. The unease intensifies as Dyer finds himself in the hospital, only to be later found murdered himself, with the chilling phrase "IT'S A WONDERFULL LIFE" scrawled using his blood. Each crime scene presents a baffling twist: the fingerprints do not correspond, suggesting multiple perpetrators. Kinderman expresses his trepidation regarding a notorious serial killer known as "The Gemini," played by Brad Dourif, who was executed fifteen years prior. This killer notoriously removed the right index finger of each victim and marked them with the Gemini sign.
During his investigation, Kinderman learns of a mysterious patient in a psychiatric ward named Dr. Temple, portrayed by Scott Wilson. He discovers that this man, once wandering in amnesia, has recently become violent and claims to be the Gemini Killer. The man bears an uncanny resemblance to Father Damien Karras, played by Jason Miller, who has been deceased. However, this patient insists he is the very essence of the Gemini Killer, ignorant of Karras's identity and proud of killing Dyer.
The plot thickens when, the following day, a nurse and Dr. Temple turn up dead. The patient in Cell 11 reveals, to Kinderman's horror, that after the Gemini’s execution, his spirit invaded Karras's dying body. The malevolent demon Pazuzu, known for possessing Regan MacNeil, is enraged by being expelled and seeks vengeance by transferring the Gemini Killer's spirit into Karras. Each night, the Gemini trades places with elderly patients suffering from dementia to carry out his heinous acts.
In a shocking turn of events, the Gemini posseses an old woman, who attempts to murder Kinderman's daughter, Julie. The fight escalates when the old woman attacks Kinderman, retreating only when Father Paul Morning, played by Nicol Williamson, steps in to perform an exorcism. However, Pazuzu intervenes, resulting in the priest's devastating defeat. Kinderman, witnessing the chaos, intends to put Karras to rest upon finding Morning's lifeless body. In a gripping confrontation, Karras momentarily regains his consciousness and pleads, “Bill, now, shoot now, kill me now!” Kinderman pulls the trigger without hesitation, mortally wounding Karras.
As Karras welcomes liberation from the Gemini’s grasp, he breathes his last words, “We won, Bill, now free me.” Ultimately, Kinderman stands somberly over Karras' grave, the film drawing to a haunting conclusion with lingering questions of morality and redemption.
