Craig Lowell prepares to leave home for his daily job as a physics teacher at the local university. His wife Joanna, who is planting green carnations, gives him one for his lapel and tells him that she’ll be taking the visiting Aunt Ada for a ride. As Craig departs, Aunt Ada waves to Joanna from her upstairs bedroom window.
As Craig arrives on campus and goes to his classroom, Professor Nick Porteus greets him and notices the green carnation. He talks about how the legends tell that burning it can repel witches, but Craig dismisses his apparently rambling. Porteus invites him to come to his office sometime to discuss the matter further.
That night, Craig wakes up at a noise and goes downstairs to find Joanna and Ada enjoying tea. Ada talks about how nature provides for many needs and offers Craig a cup of her homebrewed tea. He refuses, but secretly takes some of the herbs from her bag. Later, he has a college chemist, Frank Heller, identify them. Heller says that they’re seaweed, with no medicinal properties. However, he does determine that they were referred to in ancient times as witches’ weed. Heller suggests that Craig talk to Porteus, an expert on the subject. When Craig does so, Porteus explains that witches’ weed was used by a witch to transfer her body into a new, younger shell, and was administered in regular doses over three weeks. The ceremony must then be completed on midnight on the first autumnal equinox. Craig dismisses it as nonsense, but Porteus advises him to wear the green carnation at all times since it’s a potent anti-witch remedy.
Next, Craig goes to the graveyard and discovers that Ada Burn Quigley has apparently been dead for several months. The caretaker says that nobody pays for the grave’s maintenance, and that the family that did attend the burial all seemed remarkably surprised. He shows Craig the grave and is surprised to see that where once there was barren ground since Ada was buried, now the ground is covered in flowers.
Craig returns home and finds the ground beneath Ada’s outside chair covered in a circle of dead grass. He throws the chair into the pond and goes inside, and finds Ada alone. She says that Joanna has gone into town on an errand, and he confronts her with what he’s discovered about her supposed grave. Ada points out that Joanna has shown Craig photos of her, but he notes that their blurry and could be anyone. Further, if she really is Ada, then she’s not in her grave. Craig commands her to identify herself, but she simply laughs maniacally and creates multiple projections of herself, shocking him.
Ada leaves the kitchen and goes to her room, and begins an incantation. As Craig prepares to call Porteus for advice, the older professor is stricken down in his home by a stroke. Craig calls and Porteus’ housekeeper informs him of what happened. As Craig hangs up, he sees Ada on the upstairs staircase, watching him in satisfaction.
Later, Joanna comes home and tells Craig that Ada has said that she’ll be leaving in two days to retire to a senior citizen home. Craig realizes that the equinox is in two days, and asks Joanna to stay by him at all times until Ada is gone.
Ada continues to work magic in her bedroom. At the same time, Craig receives a telegram from a professor at the college, asking Craig to fill in for him due to a family emergency. Craig figures that it’s a trick and starts to call the college to say he won’t teach the class, but Joanna stops him. When she demands an explanation, Craig tries to describe his suspicions. Joanna doesn’t believe him, but agrees to go with him to the class so he can watch her at all times.
As the class begins, Ada inscribes a pentagram on her bedroom carpet, and watches as it ignites. In the classroom, Joanna sits in the back and begins to have flashes of the pentagram, and Ada chanting a spell. Compelled, Joanna leaves the classroom while Craig focuses on the board. Minutes later, Craig realizes that Joanna has gone and it’s close to midnight, and quickly drives for home.
When Joanna arrives home, Ada insists that she drinks one last cup of tea. Joanna tries to resist but is compelled to drink the last of the witches’ weed. When she tries to call out to Craig, Ada tells her that she won’t be needing her husband anymore.
As the clock tolls midnight, Craig arrives in the kitchen and finds Ada, unconscious. Unable to rouse her, he runs upstairs to the bedroom, where Ada is completing her spell. The witch promises him that she has Joanna, but Craig refuses to let her pass. Ada casts another spell of duplication, and she is easily able to run past Craig and go to the kitchen to complete the spell. Craig tries to get to his wife, surrounded by the witch and her illusory doubles, and is thrown back. Remembering Porteus’ words, Craig sets the green carnation on fire and throws it at Ada. As she tries to complete the final incantation, she screams in agony and is burned into ashes. Craig runs to his wife’s side as she wakes up, seemingly unharmed.
The next day, Craig goes to class and kisses Joanna goodbye. As he leaves, she very carefully avoids the green carnations in the garden.
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