Jean (Brenda Joyce) takes the job of caretaker/companion (before the word took on a completly alternate-life style meaning) to blind woman Zenobia (Gale Sondergaard.) Also hanging around the house, in this horror/western, is Mario (Rondo Hatton), a deaf-mute servant who evidently wasn't much help to Zenobia when it came to identifying the source of a noise Zenobia couldn't see. Jean is a little slow in realizing that Zenobia is slowly killing her by taking her blood. Nothing personal. Zenobia needs her blood to feed some plants. She uses the blossoms of the plants to make poison to kill cattle in order to drive away the local ranchers so she can buy all the land...cheap.

Jean Kingsley arrives in the rural town of Domingo, Nevada, to take up a new job as companion to blind scientific researcher Zenobia Dollard. At the train station Jean is surprised to run into Hal Wentley, a college beau, who still cherishes hopes about a relationship with her. Hal drives Jean out to Zenobia's isolated house, and once there introduces Jean to Mario, Zenobia's disfigured, mute servant. Although Zenobia graciously welcomes Jean, the younger woman finds herself inexplicably uneasy about her new post. Unknown to Jean, a sleeping draught has been placed in the glass of milk Zenobia insists that she drink before retiring, causing her to sleep heavily. The next night Jean is again unknowingly dosed with the sleeping draught and is oblivious when Zenobia, who has feigned her blindness, comes to her room and draws blood from her. Afterward, Zenobia and Mario go to the greenhouse, where they feed spiders to several varieties of carnivorous plants and Jean's blood to Zenobia's prize drochenema plant. Zenobia then takes some of the drochenema's petals and makes a mysterious paste from them. The following morning Jean awakens exhausted and achey, but is nevertheless curious about the abrupt departure of her predecessor, Betty Sanders, and writes her a letter. While running errands for Zenobia in town, she learns from the general store owner, Bill Stapleton, that several of Domingo's cattlemen are upset because their cattle have been inexplicably stricken. Speculation abounds that some form of weed poison is causing the cattle to die and the townspeople are also concerned about a child who became ill and died after drinking milk from the local cows. Some days later at the general store, Mr. Stapleton tells Jean that her letter to Betty has been returned as undeliverable and that the town remains in an uproar over the strange cattle deaths. He adds that several disgruntled cattlemen are leaving Domingo as their cattle continue to die without apparent cause and land prices have plummeted sharply since their deaths. On her way out of the store, Jean runs into Hal, who introduces her to Mr. Moore of the Department of Agriculture, who is investigating the cattle deaths. Later at the Dollard house, Jean comes into a room unexpectedly and finds her employer feeding a bug to a spider and, realizing she is not blind, faints in terror. Upon reviving, however, Jean is careful not to let Zenobia know she is aware she can see. Meanwhile in town, the cattlemen gather to angrily demand that Moore solve the mysterious cattle deaths. When Moore finds out from Hal that Zenobia's family used to own all the land surrounding Domingo, the two go to the Dollard house. Just before their arrival, Zenobia, realizing Jean is aware of her deception, reveals to her that she has created a poison from her drochenema flowers to drive everyone off the land she considers her inheritance, but which her father had gambled away. She admits to murdering Betty when she grew too weak to supply her venomous plant and tells Jean that she, too, will soon die. Just then Hal and Moore arrive and Zenobia greets them alone. She tells them that she knows of no indigenous poison weeds in the area and adds that Mario has just driven Jean to the train station. After taking their leave, Hal remains suspicious and, looking about the property, discovers the car still in the garage. Zenobia, watching from the window, realizes both men will return shortly and hastily orders Mario to help her burn all the evidence of the poison. The fire quickly burns out of control, attracting Hal, who bursts in and saves Jean, while Zenobia and Mario perish.