A seemingly endless series of grisly killings that bear the trademark of the mass murderer, the Gemini Killer, terrorise the district of Georgetown. To further complicate matters, even though it's been seventeen long years since the killer's execution and that fateful night of pure terror in The Exorcist (1973), the sceptical police officer, Lieutenant William F. Kinderman, is still obsessed with solving the baffling case, as the death toll keeps rising. In the meantime, in the city's high-security psychiatric institution, a cryptic inmate who bears an uncanny resemblance to the late Father Damien Karras emerges from his deep catatonic state, claiming that he has all the answers Kinderman needs. But, who is the mysterious Patient X? Does the same unholy force that tormented Regan MacNeil have something to do with the brutal demonic murders?