In England, Professor Harrington (Maurice Denham) visits his rival, Dr. Julian Karswell (Niall MacGinnis). Harrington promises to cancel an investigation of Karswell's involvement in Satanism if Karswell will rescind a threat he has made against Harrington. After learning that a parchment given to Harrington has disintegrated, Karswell glances at the clock and ushers Harrington out, promising to do all that he can. As Harrington drives home, a gigantic demon materialises and pursues him. Losing control of the car, Harrington crashes into an electrical pole and is electrocuted.Dr. John Holden (Dana Andrews) arrives in England to attend a convention at which Harrington had intended to expose Karswell's cult. Holden is informed of Harrington's death and that the only link between it and Karswell's cult is an accused murderer, Rand Hobart (Brian Wilde), who has fallen into a catatonic stupor. While Harrington's collaborators consider the possibility of supernatural forces, Holden rejects the idea as superstition.Following Harrington's notes, Holden visits the British Museum's library to examine books on witchcraft. A book Holden requests is discovered to be missing. Karswell offers to show Holden his own copy at his mansion. At Harrington's funeral, Holden meets the dead man's niece, Joanna (Peggy Cummins), who gives him Harrington's diary. It reveals Harrington's increasing fear of Karswell's power. Holden remains sceptical, but goes with Joanna to Karswell's mansion the next day.There, Holden and Karswell mock each another's beliefs. A very strong windstorm abruptly starts, which Karswell claims to have created with a spell. When Holden continues to mock him, Karswell grows angry and predicts that Holden will die in three days.Holden and his colleagues discuss Karswell and make plans to further examine Rand Hobart. Harrington's diary mentions the parchment passed to him by Karswell; Holden finds a parchment with runic inscriptions that Karswell secretly passed to him at the library. Powerful winds come through the window, blowing the parchment from his fingers. It nearly burns in the fireplace before Holden rescues and pockets it.Holden begins to feel more uneasy after a visit to Hobart's family. As Holden leaves, the parchment flies from his hand again. Hobart's family become fearful and declare Holden to be "chosen". Holden compares the parchment's runes to ones inscribed on the nearby stone circle at Stonehenge.Joanna takes Holden to Karswell's mother (Athene Seyler), who has arranged a sÃ©ance. The medium channels Harrington, who tells them that Karswell has the key to the problem from his book. That night, Holden breaks into Karswell's mansion to examine the book. He is caught by Karswell, but is permitted to leave. Holden leaves through the woods and is chased by a ball of smoke. On exiting the forest, Holden finds that the phenomenon has vanished. He reports the event to the police, but feels embarrassed.Mrs. Karswell telephones Joanna, imploring her to tell Holden that Rand Hobart knows the secret of the parchment. While Holden prepares an experiment to break Hobart's stupor, Karswell kidnaps Joanna to prevent her from giving Holden the message.Under hypnosis, Hobart reveals that he was "chosen" to die by having a parchment with a curse passed to him, but avoided death by passing it along to another person. When Holden shows Hobart the parchment he received from Karswell, Hobart goes berserk and throws himself from a window to his death.Informed that Karswell is leaving London by train, Holden races to catch it. He finds Karswell with Joanna. Karswell avoids any contact with Holden to guard against the parchment being passed back to him and grows increasingly fearful. When the train stops at the next station, Karswell tries to leave, but Holden manages to sneak the parchment into his coat. Karswell becomes frightened when he realises this. When the parchment flies from his hands, he chases it down to the tracks, but the parchment burns up. As an oncoming train approaches, a demon appears above it, seizes Karswell, slashes him, and tosses his body onto the tracks. The station crew find his mangled, smoking corpse and believe that he was struck by the train.
Output: Question: What kind of books did Holden go to the Library for?, Answer: Books on witchcraft.