One night, professor Henry Harrington, a scientist investigating a devil cult run by Dr. Julian Karswell, drives to Karswell’s country estate outside London to beg him to retract a curse he has invoked on the professor. After Harrington promises to recant his exposé, Karswell asks him to return a piece of parchment he was given that contains runic symbols. Harrington responds that the parchment flew out of his hands into the blazing fireplace at his house where it was incinerated. Later, as Harrington pulls into his garage, a demonic figure emerges from a stand of trees. Panicked, Harrington backs his car into an electrical pole and is electrocuted by the fallen lines. The next day, John Holden, an American authority on paranormal psychology, arrives in London to address a conference of his peers. Holden is met by Harrington’s aide, Lloyd Williamson, who informs him that Harrington was found dead that morning. At Holden’s hotel room, Williamson introduces the other participants of the conference—Professor Mark O’Brien, an expert in devil worship and Kumar, a psychologist from India. When O’Brien shows Holden several sketches of demons drawn by Rand Hobart, a member of Karswell’s cult who has been jailed for murder, and then suggests that the sketches prove Hobart was acting under forces of evil, the skeptical Holden scoffs at his theory. After Karswell phones to ask Holden to call off his investigation into the cult, however, Holden changes his mind about O’Brien’s theory and decides to question Hobart. The next day at the British Museum, Holden retraces Harrington’s research and discovers that a critical manuscript about witches and demons is missing. Karswell then unexpectedly appears and invites Holden to his estate to peruse his personal copy of the manuscript. After “accidentally” pushing Holden’s papers off the table and onto the floor, Karswell picks them up, then hands Holden his business card. Once Karswell departs, Holden examines the card on which the handwritten message “In memoriam Henry Harrington, allowed two weeks” has been inscribed. When Holden shows the card to the librarian, however, the handwriting has vanished. At Harrington’s funeral, Holden meets the professor’s niece, Joanna who asks to talk with him later that night at his hotel. There, she warns of danger and reads an entry from her uncle’s diary concerning the parchment given to him by Karswell. When the parchment flew out of his hands and into the fire, the professor was convinced that he had been put under a witch’s spell. Although Holden ridicules the idea of witchcraft, his curiosity has been piqued by the disappearing ink on Karswell’s card, so he asks Joanna to drive him to the Karswell estate the next day. While strolling the estate grounds with Holden, Karswell demonstrates his powers by calling down a violent wind storm. After lightning strikes a tree next to Holden, Karswell predicts that he will die on the 28th of that month at 10 p.m. Karswell then agrees to rescind the curse if Holden terminates his investigation. Unperturbed by the death threat, Holden makes a date with Joanna for dinner the next evening. Back in his hotel room, Holden whistles a melody that begins running through his head and O’Brien and Kumar identify it as a folk tune associated with the devil. When O’Brien examines Holden’s schedule book, he is puzzled by the fact that all the pages past the 28th are missing. Over dinner the following evening at the Harrington house, Joanna reads Holden an entry from her uncle’s diary that notes all the pages from his calendar beyond the 22nd, the day he died, had been torn out. As Joanna warns that the parchment sealed her uncle’s death, Holden finds a parchment of runic characters among his papers. The parchment flies out of his hands toward the fireplace, but gets caught in the grate and falls to the ground. The next day, Hobart obtains permission from Mrs. Hobart to hypnotize her son, then returns to the hotel to find Joanna waiting for him there. Joanna asks Holden to accompany her to meet Dr. Karswell's mother, who has arranged a séance with the medium Mr. Meek. At the séance, after going into a trance, Meek begins to speak in Harrington’s voice. When Harrington, through Meek, tells Holden that he must seek out the manuscript that Karswell has translated, Joanna insists on driving to Karswell’s estate to search for the translation. Holden sneaks into the library and locates the translation, but is prevented from examining it when Karswell’s house cat transforms himself into a jaguar and attacks. When Karswell enters the room, the jaguar changes back into a cat. After exchanging hostile words with Karswell, Holden angrily departs. As he crosses the woods on the way to Joanna’s car, a ball of smoke suddenly materializes, expands into a cloud and then disappears. Joanna drives Holden to Scotland Yard to report the sighting, but once there, he debunks the cloud as “just a trick gadget rigged by Karswell.” Insulted by Holden’s insinuation that she has “stampeded him into hysteria,” Joanna leaves in a huff. On the 28th, Holden proceeds to the conference where he is scheduled to discuss hypnosis. Using Hobart as his subject, Holden puts him into a trance and shows him the sliver of parchment. Hobart admonishes that to save his life, Holden “must pass the parchment back to the one who gave it to him, who will then be taken by the demon.” Then, overwrought by the incantation, Hobart runs to a window and plunges to his death. Now realizing that to save himself, he must hand the parchment to Karswell before 10 p.m. that night, Holden tracks the doctor to the train station where he is scheduled to leave on the 9:45 train to Southampton. After jumping aboard the departing train, Holden locates Karswell’s compartment. Joanna, who has been hypnotized by Karswell, is also there. After Karswell releases Joanna from her trance, Holden offers him a note of apology and a cigarette, all rejected by Karswell, who fears they may contain the parchment. As Karswell nervously rushes for the door, Holden hands him his coat, the parchment stuffed into a pocket. As the parchment flies out of the pocket and out the stopped train, Karswell chases it along the tracks. Some distance away,the parchment transforms into a demon, then snatches Karswell off the tracks, crushes him and throws him into the path on an onrushing train. When the police find Karswell’s mutilated body, they conclude that his death was caused by the impact of the train. Holden then turns to Joanna and states "sometimes it is better not to know."