American psychologist John Holden (Dana Andrews) arrives in England to discover that his colleague Henry Harrington (Maurice Denham) has suddenly died following his efforts to discredit notorious occultist Julian Karswell (Niall MacGinnis). The cynical Holden dismisses Karswell's warnings as supernatural nonsense even when he and Harrington's niece Joanna (Peggy Cummins) are confronted by a series of bizarre and inexplicable events. Holden discovers that Karswell has slipped him a parchment featuring ancient runic symbols a sign that like Harrington before him he has been marked for imminent destruction by a fire-breathing demon. As the night of his threatened demise draws nearer the fearful Holden begins to suspect that Karswell has been telling the truth all along...