Father Philip Lamont (Richard Burton), who is struggling with his faith, is assigned by the Cardinal (Paul Henreid) to investigate the death of Father Merrin (Max Von Sydow), who had been killed in the course of exorcising the Assyrian demon Pazuzu from Regan MacNeil (Linda Blair). While Lamont, who's had some experience at exorcism, thinks of Merrin as a saint, he is actually up on posthumous heresy charges. Some Church authorities are not sure the exorcism should have been performed (even though it was officially approved by the local Bishop). Merrin’s writings are considered very controversial. Apparently, Church authorities are trying to modernize and do not want to acknowledge that Satan (in terms of an actual evil entity) exists.
Although now seemingly normal and staying with guardian Sharon Spencer (Kitty Winn) while her mother is on location, Regan continues to be monitored at a psychiatric institute by Dr. Gene Tuskin (Louise Fletcher). She claims she remembers nothing, but Tuskin believes her memories are only buried or repressed. In an attempt to plumb her memories of the exorcism, specifically the circumstances in which Merrin died, Dr. Tuskin has hypnotized the girl, to whom she is linked by a "synchronizer" -- apparently a kind of biofeedback device that is used by two people to synchronize their brainwaves. Tuskin finds herself telepathically "witnessing" Regan's memory of the event. She is attacked by Pazuzu and Father Lamont has to use the synchronizer to rescue her.
After a guided tour by Sharon of the Georgetown house where the exorcism took place (wherein Sharon confesses to leaving the MacNeils for two years before coming back, claiming she is never at ease unless she remains near Regan), Lamont returns to be coupled with Regan by synchronizer. The priest is spirited to the past by Pazuzu to observe Father Merrin exorcising a young boy, Kokumo (Joey Green), in Africa. Learning that the boy developed special powers to fight Pazuzu, who appears as a swarm of locusts, Lamont journeys to Africa, defying his superior, to seek help from the adult Kokumo (James Earl Jones).
Lamont learns that the reason Pazuzu attacks certain people is that those people all have some form of psychic healing ability. The exorcism he performed at the beginning of the film was for a South American lady who said she "healed the sick". Kokumo has since become a scientist, studying how to prevent locust swarms from attacking native crops. Regan, possibly taking a cue from her experience with the synchronizer, is able to reach telepathically inside the minds of others; she uses this to help an autistic girl to speak, for instance. Father Merrin belonged to a group of theologians who believed that psychic powers were a spiritual gift which would one day be shared by all humanity in a kind of global consciousness (akin to the ideas of Teilhard de Chardin, on whom Blatty originally based Merrin's character); he thought people like Kokumo and Regan were foreshadowers of this new type of humanity. In a vision, Merrin asks Lamont to watch over Regan.
For some reason, this necessitates Lamont and Regan returning to the old house in Georgetown where she was possessed. The pair are followed by Tuskin and Sharon, concerned about Regan's safety. En route, Pazuzu tempts Lamont (and apparently Sharon) by offering them unlimited power. Lamont resists and continues with his original plan. In the house, a swarm of locusts deluge the pair and the entire house begins to crumble around them. Pazuzu appears as a kind of tarted-up version of Regan herself, and Lamont has to resist this temptation as well -- by beating open its chest and pulling out its heart. Once he's done this, Regan banishes the locusts (and, one assumes, Pazuzu) by enacting the same ritual used by Kokumo to get rid of locusts in Africa. Outside the house, Sharon is apparently possessed by Pazuzu, but kills herself. Tuskin tells Lamont to watch over Regan and the pair leave; Tuskin remains at the house to answer police questions.
The film ended up becoming one of the most notorious commercial failures ever released. Because reports indicated that the film inspired audience laughter at its premiere, prints were hastily pulled from release and quickly recut by Boorman in an attempt to make the film's narrative more comprehensible. The revised version fared no better (some critics commented that Boorman's restructuring made the film even more incoherent) and was the only version of the film available for many years, until the release on videocassette of the original cut in the early 1990s.
Years later, Boorman commented on the film's re-editing: “The sin I committed was not giving the audience what it wanted in terms of horror…There’s this wild beast out there which is the audience. I created this arena and I just didn’t throw enough Christians into it. People think of cutting and re-cutting as defeat, but it isn’t. As Irving Thalberg said: ‘Films aren’t made, they’re remade.’”
Jon Voight, David Carradine, Jack Nicholson and Christopher Walken all were considered or offered the part of Father Lamont, who John Boorman initially conceived as a younger priest in awe of Father Merrin's writings. Voight agreed to sign on, but later dropped out over script concerns. David Carradine was locked in a financial dispute with his television show Kung Fu, and Jack Nicholson's salary was deemed too high. Eventually the choice was made to age the character, and Richard Burton was signed for the role. Linda Blair claims Burton was charming and likable on-set, but drank heavily the longer the shoot went on.
The role of Dr. Tuskin was originally written for a man, with Chris Sarandon and George Segal both considered. When the gender of the character was changed, both Ann-Margret and Jane Fonda were under consideration.
Originally, the script had a major role for Lee J. Cobb's character of "Lieutenant Kinderman" from the first film, but upon his death the story was reworked. The script for The Heretic apparently bore no resemblance to William Peter Blatty's Legion, which was later made into The Exorcist III and featured Lieutenant Kinderman (then played by George C. Scott) in a starring role.
The film reportedly damaged Blair's career, who went from being one of Hollywood's hottest young actresses to being relegated to low-budget exploitation films. Boorman recovered from the film's financial failure with the successful Excalibur a few years later, and the Exorcist series would eventually get a more conventional, audience-friendly sequel The Exorcist III (1990), adapted by William Peter Blatty from his own novel Legion.
Bob McCabe's The Exorcist: Out of the Shadows (Omnibus Press, 1999) contains a chapter on Exorcist II: The Heretic in which William Peter Blatty, William Friedkin and Linda Blair noted their negative responses to the sequel:
Also, throughout Exorcist II, Father Merrin is frequently mentioned by the characters as having saved Regan, yet no mention is made of Father Karras, who in fact did save Regan.
One of the key elements of Exorcist II: The Heretic is Father Merrin's exorcism of a young boy named "Kokumo" in Africa. This exorcism is first referenced in the original film The Exorcist, and actually illustrated with flashbacks in Exorcist II: The Heretic. Although this same exorcism becomes the central plot line for the most recent Exorcist movies Dominion: Prequel to the Exorcist and Exorcist: The Beginning, little effort was made to keep the stories consistent.
The chapel scenes were cleary filmed in Ethiopia and the priests can be heard speaking Amharic. The setting resembles almost exactly the Ethiopian Orthodox Church and appears to be set in the northern province of Tigray where rock churches are well known. However when father Merrin searches for Kokumo in the village, he is given directions in French.