AmericanpsychologistJohnHoldenarrivesinLondontoparticipateinasymposiumaimingtoexposewitchcraftanddevil-worshipasafraud,withattentioncenteredonacultrunbyaJulianKarswell.Debarkinghisplane,Holdenlearnsthatthesymposiumleader,HenryHarrington,hassuddenlydiedandheisnowincharge.WhatonlytheaudienceknowsisthatHarringtonwasclearlykilledbyademon,apparentlysummonedbyKarswelltoavengehispersecution.DespiteboththreateningandcajolingbehaviorbyKarswell,Holdendeterminestoproceed,thoughtheonlycultmemberwillingtotalkabouthisexperiencesisinaprisonpsychiatricward,catatonicafterallegedlykillingyetanothercultmember.However,Harrington'sniece,Joanna,believes,basedonheruncle'snotes,thatKarswellmayindeedhavesomesatanicpowersderivedfromanancienttomeinanobscurelanguagewhichhehasbeenabletotranslate,andattemptstowarntheskepticHolden.Karswell'smotheralsoattemptstowarnHoldenoff.However,thethingwhichbeginstomostgettoHoldenaretheoccasional,butmounting,sensoryandmentaldisturbancesheb