Georgetown, Washington D.C. Actress and divorced mother Chris MacNeil starts to experience 'difficulties' with her usually sweet-natured eleven-year-old daughter Regan. The child becomes afflicted by spasms, convulsions and unsettling amnesiac episodes; these abruptly worsen into violent fits of appalling foul-mouthed curses, accompanied by physical mutation. Medical science is baffled by Regan's plight and, in her increasing despair, Chris turns to troubled priest and psychiatrist Damien Karras, who immediately recognises something profoundly malevolent in Regan's distorted fetures and speech. On Karras' recommendation, the Church summons Father Merrin, a specialist in the exorcism of demons . . .

One by one the bizarre murders frustrate and torment Lt. Kinderman, the homicide detective from The Exorcist. A boy, crucified; a priest, decapitated; another priest slain; a nurse, slaughtered—all bear the Zodiac mark of the Gemini Killer. BUT ... the Gemini Killer has been dead for 12 years—Lt. Kinderman stalks the brutal and elusive killer down the dark streets. Until,finally, in desperation he dares to cross the boundary that separates the living from the dead.