On a trip to meet his girlfriend's family, a young man uncovers deadly secrets.

An American scientist, Stephen Reinhart, goes to the village of Arkham, England, to visit Susan Witley, the woman he hopes to marry, and to meet her parents. He is put off by the villagers' lack of warmth, by the bleak grounds surrounding the Witley house, and by the cold reception given him by Susan's father, Nahum. Susan's mother, the bedridden and veiled Letitia, greets him warmly, however, and pleads with him to take Susan away. That night he encounters frightening apparitions and sees from his window the burial of Nahum's manservant Merwyn. The next day he goes to the village to find out about the strange occurrences at the Witley house, but he learns nothing. Later, Stephen and Susan investigate the Witley greenhouse, from which an unearthly glow emanates, and there they discover grotesque plants and organisms. In the cellar of the Witley house they find a huge, radioactive meteorite, which Nahum has been using to create mutations. Letitia, already disfigured from overexposure, is subsequently killed; and Nahum, also exposed to the radiation, dies in a fire which engulfs the house as Susan and Stephen escape.