As Craig Lowell leaves for his scientific teaching job at campus, he talks to his wife Joanna, who is planting roses. When he asks what she's going to do with her day, Joanna says that she'll take Aunt Ada for a ride. Craig seems surprised and drives off, while Aunt Ada waves to Joanna from her upstairs window.
At campus, Professor Nick Porteus notices that Craig is wearing a green carnation, and comments that it has supernatural significance. Craig hastily dismisses his interest, but Porteus suggests that he come to his rooms some time to discuss the matter further.
One night, Craig wakes up and discovers that Joanna and Ada are having tea because they couldn't get to sleep. Ada explains that nature provides for everything and offers him a cup, and Craig turns down her offer. As she talks, Craig takes some from the herbs from her tea bag. The next day he takes them to a chemist, Frank, who identifies the herbs are seaweed. They have no medicinal properties, but they were once known as witches' weed. Frank has no idea what it is, and suggests that Craig talk to Porteus. Porteus tells Craig to handle it with care, and explains that an aging witch uses it to enter a new, younger body that she has chosen for herself. On midnight on the first autumnal equinox, the witches' weed must be administered in doses over three weeks. Craig dismisses it as nonsense, but Porteus notes that a green carnation is a powerful anti-witches charm.
Craig goes to a graveyard and examines the grave of Ada Burn Quigley, who was buried six weeks ago. The caretaker says that the family that attended all looked similar. He's surprised to discover that flowers are now growing on the grave, where nothing has grown before.
Later, Craig returns home and notices a patch of dead grass beneath Ada's chair. He throws the chair into the pond and goes inside, and Ada says that Joanna has gone into town. Craig tells her that he went to great-aunt Ada's grave, and asks who she is. He points out that the only evidence that she is who she claims is a fuzzy photo, and if she is Aunt Ada, then she's not in her grave. Craig demands to know who she is and what she wants, and Ada laughs maniacally and then creates multiple images of herself.
In her room, Ada begins chanting and gesturing. At his home, Porteus suffers from a stroke. Craig calls seconds later and the housekeeper informs him of what happened. After Craig hangs up, he sees Ada watching him from the stairway.
Later, Joanna finds Craig digging in the garden. She tells him that Ada has announces that she'll be leaving in two days to go to an old folk's home. Craig realizes that the equinox is in two days and asks Joanna not to leave his side until the two days have passed.
Ada continues to chant in her room. Downstairs, Craig receives a telegram from a colleague who has been called out of town. He wants Craig to take over his that night, but Craig realizes that it's a trick. He starts to call the college to say he can't teach the and Joanna demands to know why he's acting strangely. Craig explains that a witch took over Ada's body, and she plans to take Joanna's body at midnight. Joanna doesn't believe it, but says that she'll go with Craig so he can keep an eye on her.
That night, Ada inscribes a burning pentagram on her carpet. Meanwhile, Joanna watches from the back of the as Craig lectures. She starts to have flashes of Ada and the pentagram, and the witch casting a spell of transference. Joanna clutches at her head in pain and then slips away to drive home.
As the clock approaches midnight, Craig realizes that Joanna has gone. He runs to his car and drives home.
Joanna enters the house and Ada greets her with a cup of tea. She says it's the last mouthful that is necessary, and Joanna backs away in terror. After a moment she stops and is compelled to take the cup. She sits down and calls out to Craig, but Ada says they won't be needing him anymore.
As the clock starts to toll midnight, Ada looks outside and sees Craig running to the house. He calls out to Joanna and finds her in the kitchen, unconscious. Seeing the cup of tea, he runs up to Ada's room and finds her crouched over the pentagram. The witch boasts that Joanna is hers, but Craig vows not to let her past. Ada casts a spell of duplication. As Craig runs downstairs, he finds himself helpless to penetrate the witch's illusion. However, remembering Porteus' words, he sets the green carnation on fire. The flames consume Ada as she chants the last few words. Craig runs to Joanna's side as she wakes up.
Later, Joanna kisses Craig goodbye as he drives off to class. However, once he's gone, "Joanna" carefully avoids the green carnations growing in the garden.