American psychologist John Holden arrives in London to attend a symposium dedicated to unveiling witchcraft and devil-worship as fraudulent practices, focusing on a cult led by Julian Karswell. Upon disembarking his plane, Holden discovers that the symposium's leader, Henry Harrington, has unexpectedly passed away, leaving him in charge. Unbeknownst to Holden, the audience is aware that Harrington was killed by a demon, seemingly summoned by Karswell as retribution for his persecution. Despite Karswell's attempts to intimidate and persuade him, Holden is determined to continue his mission. The only cult member willing to discuss his experiences is held in a prison psychiatric ward, rendered catatonic after allegedly killing another cult member. Harrington's niece, Joanna, believes from her uncle's notes that Karswell may possess satanic powers derived from an ancient, untranslated tome, and endeavors to caution the skeptical Holden. Karswell's mother also tries to dissuade Holden. However, Holden starts experiencing occasional but growing sensory and mental disturbances following an encounter with Karswell. During this meeting, Karswell discreetly slipped a piece of parchment with a curse written in runic symbols into Holden's belongings—a parchment identical to one Harrington had received in a similar fashion before his untimely demise. [N.B. Holden never stays at the Karswell estate].