恶魔之夜American psychologist John Holden arrives in London to participate in a symposium aiming to expose witchcraft and devil-worship as a fraud, with attention centered on a cult run by a Julian Karswell. Debarking his plane, Holden learns that the symposium leader, Henry Harrington, has suddenly died and he is now in charge. What only the audience knows is that Harrington was clearly killed by a demon, apparently summoned by Karswell to avenge his persecution. Despite both threatening and cajoling behavior by Karswell, Holden determines to proceed, though the only cult member willing to talk about his experiences is in a prison psychiatric ward, catatonic after allegedly killing yet another cult member. However, Harrington's niece, Joanna, believes, based on her uncle's notes, that Karswell may indeed have some satanic powers derived from an ancient tome in an obscure language which he has been able to translate, and attempts to warn the skeptic Holden. Karswell's mother also attempts to warn Holden off. However, the thing which begins to most get to Holden are the occasional, but mounting, sensory and mental disturbances he begins experiencing after an encounter with Karswell, and the discovery that, during this encounter, Karswell secretly slipped a slip of parchment with a curse in runic symbols on it into Holden's belongings, a parchment identical to one Harrington recorded having had passed to him in a similar manner shortly before his death... [N.B. Holden at no point stays at the Karswell estate].