Die, Monster, Die!

Stephen Reinhart travels to Arkham to visit his fiancée Susan Witley. He is met coolly by Susan's father Nahum, while Susan's bedridden mother Letitia appears welcoming. A box containing a gold earring, claimed to belong to the missing Helga, is thrust into Stephen's hands, hinting at the estate's unsettling past.

During dinner, Stephen asks about a blackened patch of land near the estate, which Nahum and Susan attribute to a fire but cannot fully explain. Almost immediately, Mervyn the butler collapses at the table, and a cloaked figure is seen peering in at the window. The unsettling events raise questions about the estate's hidden dangers.

That night, Stephen and Susan hear mysterious noises from the basement and are confronted by Nahum, who nervously reveals that Mervyn has died. Stephen then follows Nahum into the woods and watches him bury Mervyn's body, confirming that something sinister is at work. A strange light glows from the greenhouse, suggesting experiments beyond ordinary science.

At dawn, Stephen leaves the estate but is chased briefly by a cloaked figure in the surrounding woods before it vanishes. The encounter reaffirms the sense that danger stalks the Witley household.

In the village, Stephen seeks Henderson, the town doctor, who is reluctant to speak due to his ties to the Witleys. Henderson's secretary tells Stephen that Susan's grandfather Corbin Witley died in Henderson's arms under mysterious circumstances, deepening the sense of a concealed history.

Stephen and Susan confront the truth behind the greenhouse discoveries and explore the greenhouse's interior. They find plants and flowers grown to an abnormally large size, a radiation-emitting machine, and several large cages. They also recover meteorite fragments that appear to be emitting radiation, linking the strangeness to Nahum's experiments.

Letitia and Helga are revealed to have frequently worked in the greenhouse, explaining their illnesses and disfigurements as consequences of the experiments. The clues mount that the Witley estate's lush growth comes with a terrible cost.

Nahum confronts Stephen in the basement and shows a large chamber containing a radioactive meteorite. Upstairs, Letitia's room is found empty and in disarray, signaling that she may have been drawn into the danger. The tension escalates as real peril closes in on the family.

That night, a grossly disfigured Letitia attacks Stephen and Susan, underscoring the devastating effects of the meteorite's radiation. They manage to defend themselves and begin to realize the true cost of Nahum's experiments.

Nahum explains that the meteorite fell near the heath, triggering rapid plant growth and inspiring his plan to reshape the estate's landscape. The experiment's results spiral out of control as Helga, cloaked in disguise, attacks Nahum in the basement and accidentally dies after tumbling onto the meteorite. Nahum, now heavily irradiated, shows signs of disfigurement and power over the environment.

Nahum's exposure to the meteorite leaves him burnt and grotesquely changed as he chases Stephen and Susan through the house. The mansion catches fire, and the couple narrowly makes it out to safety as flames engulf the estate.

Stephen and Susan escape the burning Witley mansion with their lives, stepping into the night as the estate is consumed by fire. The film closes on a note of unresolved consequences, with the danger of the meteorite and Nahum's experiments lingering in the background.