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.

Col. Vincent Kane is a military psychiatrist who takes charge of an army mental hospital situated in a secluded castle. Among Kane’s many eccentric patients is Capt. Billy Cutshaw, a troubled astronaut in the midst of an existential crisis. Although Kane’s own grasp on sanity is questionable, he manages to engage Cutshaw in a series of thoughtful conversations about science and faith that deeply affect the lives of both men.