The film opens with Father Philip Lamont (Richard Burton), a priest who struggles with her faith, trying to exorcise a possessed girl from South America, which seeks to “heal the sick”. Later, Lamont is assigned by the Cardinal (Paul Henreid) to investigate the death of Father Lankester Merrin (Max von Sydow), who had been assassinated four years ago while exorcising the Assyrian demon Pazuzu (Karen Knapp) in the body of Regan MacNeil ( Linda Blair). The Cardinal informed Lamont (who had some experience in exorcism, and has been exposed to the teachings of Merrin) that the Church had accused Merrin on charges of heresy by the performance of exorcism to Regan. Apparently, church officials are trying to modernize and fail to recognize that Satan (in terms of an actual evil entity) exists.Regan, although apparently normal now and his mentor Sharon Spencer (Kitty Winn) in New York, continues to be monitored in a psychiatric institute by Dr. Gene Tuskin (Louise Fletcher). Regan says she does not remember anything what happened in Washington DC, but Tuskin believe your only memories are buried or repressed. Father Lamont visit the school, but his attempts to consult with Regan about the circumstances of the death of Father Merrin is rejected by Dr. Tuskin, believing that Lamont’s question would do more harm than good to Regan. In an attempt to plumb her memories of exorcism, in particular the circumstances of his death Merrin, Dr. Tuskin mesmerize the girl, which is attached to a “synchronizer” (a kind of biofeedback device used by two persons to synchronize their brain waves). After a guided tour of Sharon from the Georgetown home, where he performed the exorcism, Lamont returns along with Regan by synchronizer.The priest is a spirit in the past where he sees the Father Merrin exorcising a young boy, Kokum (Joey Green), in Africa. Learning that the boy developed special powers to fight Pazuzu, who appears as a plague of locusts, Lamont travels to Africa, defying his superior and Kokum seeking help from an adult (James Earl Jones). Lamont Pazuzu learns that attacks all have some sort of psychic healing ability. Kokum has become a scientist, and is dedicated to the study of how to prevent locust swarms that attack native crops. Regan is able to telepathically reach the minds of others, and use this to help an autistic child. Father Merrin belonged to a group of theologians who believe that psychic powers were a spiritual gift, that one day would be shared by all humanity in a kind of global awareness. In a vision, something Merrin asks Lamont to ensure the safety of Regan.For some reason, this requires that Lamont and Regan return to the old house in Georgetown. The couple were followed by Tuskin and Sharon Regan concerned. Along the way, Lamont tempts Pazuzu appears as another Regan, offering unlimited power. Lamont resists attacks Regan’s double and a swarm of locusts attacked the couple and the whole house begins to crumble around him. But Lamont fight with Regan’s double and opens your chest and pull his heart. In the end, Regan banishes lobsters (Pazuzu as well) by enacting the same ritual used by Kokum to get rid of locusts in Africa. Outside the house, Sharon dies from burn injuries, and Lamont tells Tuskin ensure Regan. Both left and then the police arrive Tuskin who asks for what happened, but she does not answer.