In a short introduction a paranormal investigator, Dr. Harringtonton, meets with cult leader Dr Karswell, begging the satanist to "call it off", Karswell makes a half-hearted promise to "do what he can", but the scientist is killed by a demon on his way home.
Cue the bulk of the movie, in which Dr. Holden, an American scientist and friend of the now diseased investigator, travels to England to look into Harrington's death (which Holden believes to be either an accident), and himself take over the work of exposing Karswell as a fraud.