Set fifteen years after the original film, The Exorcist III centers around the philosophical Lieutenant William F. Kinderman who is investigating a baffling series of murders around Georgetown that all contain the hallmarks of The Gemini, a deceased serial killer. It eventually leads him to a catatonic patient in a psychiatric hospital who has recently started to speak, claiming he is the The Gemini and detailing the murders, but bears a striking resemblance to Father Damien Karras.

Bizarre nightmares plague Regan MacNeil four years after her possession and exorcism. Has the demon returned? And if so, can the combined faith and knowledge of a Vatican investigator and a hypnotic research specialist free her from its grasp?

Having lived through traumatizing events during WWII, Father Lankester Merrin takes a sabbatical from the Church to conduct archaeological excavations in British-administered East Africa. Merrin unearths an ancient Byzantine church believed have been built and then immediately buried to keep down evil from the crypt below. The natives are convinced that uncovering the church has unleashed a demon, and begin to violently clash with the British military troops. As the village rapidly disintegrates into chaos and war, Merrin must face-off with the demon which has taken possession of somebody close to him.