Stephen Reinhart, Nick Adams, an American scientist, travels to Arkham, England to visit his fiancée Susan Witley Suzan Farmer. At the sprawling Witley estate, Nahum Witley [Boris Karloff] greets him with cool formality, while Susan's bedridden mother Letitia Witley [Freda Jackson] offers a guarded welcome from behind her bed canopy and presents Stephen with a box containing a gold earring that she claims belonged to her maid Helga, who has fallen mysteriously ill and disappeared. The atmosphere is thick with unease as the family history seems to press in on every room the visitors enter. Over dinner, talk turns to a blackened patch of land nearby that looks decimated by some inexplicable event. Nahum and Susan offer partial explanations, but neither can fully illuminate what happened, and a sense of hidden dangers lingers in the air.
The tension mounts when the household is shaken by a series of disturbing incidents. The butler, Mervyn, suddenly collapses, and soon a cloaked figure is seen at the estate’s windows. Late at night, Stephen and Susan hear eerie sounds emanating from the basement, and Nahum nervously reports that Mervyn has died. Stephen later witnesses Nahum at the graveyard, burying Mervyn’s body in the woods, a motion that deepens the sense that something unnatural is at work. In the pale dawn, Stephen is followed by a cloaked assailant in the woods, an encounter that leaves him shaken and more determined to uncover the truth. Back in the village, Dr. Henderson [Patrick Magee] is wary of speaking with Stephen due to old ties to the Witleys, and Henderson’s secretary informs Stephen that Susan’s grandfather Corbin Witley died in Henderson’s arms, though the cause remains shrouded in mystery.
Driven to uncover the truth, Stephen and Susan venture to the greenhouse, where they find plants and flowers grown to an almost fantastical size. In a potting shed, they uncover a machine that emits radiation, and they discover several large, caged creatures. Stephen also finds fragments of meteorite stone that appear to radiate energy, while Susan notes that both her mother and Helga often worked in the greenhouse, hinting at a link between the family’s experiments and the strange afflictions that have plagued them. In the basement, Nahum confronts Stephen and the two uncover a vast chamber containing a glowing meteorite, linking the meteor to the bizarre growth and mutations around the estate.
As the investigation deepens, Letitia’s room is found empty and in disarray, and the tension between science and secrecy reaches a breaking point. The family’s precarious balance shatters when a disfigured Letitia,
now visibly altered by radiation, attacks. During the subsequent burial in the family plot, Nahum explains how the meteorite’s arrival near the heath triggered a rapid, lush growth of life around the estate and how his aim was to harness that power through mutation and radiative means. The revelation sets Nahum on a dangerous path, and when he tries to destroy the meteorite in the basement, Helga—charged with fear and zeal—attacks him with an axe. The confrontation ends tragically as Helga falls onto the meteorite, dying, while Nahum, now heavily irradiated, becomes a walking embodiment of his experiment’s grotesque consequences.
In a dramatic finale, Nahum, overwhelmed by radiation, chases Stephen and Susan through the burning mansion, which erupts into flames as the meteorite’s influence proves uncontrollable. The couple makes a perilous escape from the blazing Witley estate, stepping out into the night just as the house collapses behind them, leaving behind a landscape changed by science, superstition, and a meteor’s strange, dangerous gift. The tale lingers with a sense of caution about unchecked curiosity and the price of manipulating nature, and it cements Arkham’s reputation as a place where the boundaries between the familiar and the uncanny are perilously thin.
