William Sebastian (Robert Culp) is a former criminologist who now studies the occult to explain the problem of human evil. He has been cursed on one of his adventures by the demon Asmodeus, leaving him in constant need of medical attention. He summons an old colleague, Dr. "Ham" Hamilton (Gig Young) to his home to help him with a case involving the Cyon family. Dr. Hamilton does not believe in the occult and thinks that Sebastian and his housekeeper Lilith (Majel Barrett) are playing tricks on him when he witnesses unusual events.
As the pair are getting reacquainted, a woman claiming to be Anitra Cyon (Ann Bell) unexpectedly visits and tells Sebastian that his services are no longer required. Sebastian recognizes that this person is not Anitra Cyon, but rather a succubus sent to stop Sebastian from investigating the family. He defeats her using the Apocryphal Book of Tobit. He and Ham depart to the airport where they are flown by Mitri Cyon (John Hurt) to London. While over the Atlantic the engines of the plane fail, something that Sebastian claims is the result of supernatural intervention.
Thanks to Mitri's expertise, they land safely in England. Sebastian asks the Cyon chauffeur to stop off at the home of Dr. Qualus, a long-time associate who has been researching the Cyon family. Qualus' house is on fire. Ham and Sebastian find Qualus' body lying partially inside a pentagram, the body fatally mauled. The police arrive, and Inspector Cabell (an old acquaintance from their criminology days) escorts them to Cyon Manor. When they reach the Manor, they are greeted first by Sir Geoffrey Cyon (James Villiers), Anitra and Mitri's older brother and head of the family. He is aware that Sebastian and Ham are there to investigate him, and he intends to defy their efforts. The real Anitra (also played by Ann Bell) reiterates her reason for consulting Sebastian: due to an unknown influence, Geoffrey's behaviour has changed dramatically and he has turned Cyon Manor into a den of iniquity. Geoffrey dismisses her concerns. The following day Mitri is attacked by dogs, leaving him in critical condition. Sebastian and Ham suspect Geoffrey is behind the attack. That night, the duo investigate the grounds of the Manor and discover ruins which lead to a hidden underground temple devoted to Asmodeus. They suspect that the real Geoffrey is dead and that Asmodeus has assumed his form.
The next day the pair prepare to do battle with Asmodeus and his cult. Further investigation of the hidden temple reveals that it is Mitri who is dead and whose form Asmodeus has assumed. Geoffrey is a pawn who was used to increase the cult and who will be forced to sacrifice Anitra to Asmodeus. Sebastian defeats Asmodeus, whose power is destroyed. The Cyons and the cultists are freed, and the curse on Sebastian is removed.
In thanks for his help, the Cyon family gives Sebastian a painting which he had admired -- but in the final moment of the film, Sebastian is stunned to see the symbol of Asmodeus (a stylized letter "A") suddenly appear in the corner of the artwork. Asmodeus survives, and the conflict is only beginning.
Spectre was intended as the pilot for a television series, but was rejected. The relationship between Sebastian and "Ham" is deliberately reminiscent of that of Sherlock Holmes and Dr. Watson, although there are also some aspects that recall the relationship between Roddenberry's own Spock and Leonard McCoy. Roddenberry previously revisited this relationship in an earlier failed pilot, The Questor Tapes.
Spectre was one of a number of unsuccessful television pilots in the 1970s in the occult detective subgenre. After its rejection by American television, an extended version of Spectre was released in the UK as a theatrical film with additional footage that includes nudity. The version currently in television syndication is a heavily edited version of the UK theatrical release, which retains some of the less explicit nudity in the Black Mass finale. A novelization of Spectre by the author Robert Weverka was published in 1979.Robert Culp as William Sebastian
Gig Young as Dr. Amos "Ham" Hamilton
John Hurt as Mitri Cyon
James Villiers as Sir Geoffrey Cyon
Gordon Jackson as Inspector Cabell
Ann Bell as Anitra Cyon
Majel Barrett as Lilith
Jenny Runacre as Sydna
William Sebastian (Robert Culp) is a former criminologist who worked in partnership with Amos "Ham" Hamilton for eight years. He is a brilliant detective with incredible intuitive skills and a belief that there are things beyond science that are real. He came to believe that some unseen forces were causing a number of significant crimes, particularly after studying Charles Manson, Richard Speck, the Boston Strangler, and the Tokyo Bluebeard. After his split with Ham five years ago he immersed himself in the Occult, and has an extensive collection of occult artifacts. Recently Sebastian undertook an occult experiment which has damaged his heart.
Dr. Amos "Ham" Hamilton (Gig Young) is a medical doctor who worked as staff physician and forensic pathologist at Fairview General Hospital in Boston, Massachusetts, but thirteen years ago he started a partnership with William Sebastian as criminal investigators. He was the more down-to-earth member of the partnership and after eight years Ham became tired of working with Sebastian and returned to Fairview. Recently his alcoholism and womanizing has interfered with his work.
Lilith (played by Gene Roddenberry's wife Majel Barrett) is Sebastian's housekeeper, a practicing witch who brews a remedy that "cures" Ham's alcoholism through aversion therapy.
A novelization of the movie was written by Robert Weverka, and published by Bantam Books in 1979 (ISBN 0-553-13302-0) Robert Weverka novelised a number of other television and movie productions including The Waltons, Apple's Way, The Sting, The Magic of Lassie, and Murder by Decree. The Spectre novelisation is 154 pages long and adds significant background information not present in the script.