William Sebastian (played by Robert Culp) is a criminologist who has taken to studying the occult to explain the problem of human evil. He has been cursed on one of his adventures, leaving him in constant need of medical attention. He summons an old colleague, Dr. Hamilton (played by Gig Young) to his home to help him. The two are soon summoned to England to investigate strange happenings involving a mysterious Satanic cult and the demon Asmodeus.
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). The movie also features Roddenberry's wife Majel Barrett in a small role as Sebastian's housekeeper Lilith, a practicing witch (she brews a remedy that "cures" Ham's alcoholism through aversion therapy). Other members of the cast include John Hurt, Gordon Jackson, Ann Bell, and James Villiers.
Spectre was one of a number of unsuccessful television pilots in the 1970s in the occult detective sub-genre.
After its rejection by American television, an extended version of Spectre was released in the UK as a theatrical film. The version currently in television syndication is a heavily edited version of the UK theatrical release.