A deputy gets picked up by a young woman named Sylvia in a bar. After intercourse she starts to cry and tells him that something didn't work. Suddenly she grows old and dies.
The next day Dr. Christina Markham, the wife of the Sheriff, does the autopsy on the already strongly decomposed body. Besides Alzheimer's and several kinds of cancer, a strange anomaly in the uterus is found. Shortly afterwards a second female body is found similarly decomposed and is identified as a local woman who was over 70 years old. Both bodies show the same anomaly and both contain traces of semen.
While Dr. Markham is visiting her mother (who has Alzheimer's) in a care center, she hears her and another old woman talking about 'going into the light' soon. The other woman, the mother of the deputy, disappears from the center and visits her son as a young woman. She leaves and also has intercourse with a stranger. Afterwards she returns to her son, tells him that 'it' didn't work, grows old and dies shortly afterwards.
In the night, Dr. Markham's mother runs away from the care center together with Gerry, the brother of her late husband. She leads him to a meadow where she looks for a stone which is quickly found. The stone starts to produce a beam of light which rejuvenates both of them. After they confess their love to each other, they have intercourse in the Markham house. Meanwhile the Sheriff and his wife figure out that the three dead woman and Dr. Markham's mother were arrested together 50 years ago for trespassing at the place where the second body was found. They start a search for her and Gerry, after finding them missing from the care center. They find them in their family house: Gerry is already in his normal old age again but still looks healthy enough, while Mrs. Markham is still young but now in an advanced state of pregnancy.
She explains that she and her three friends encountered a female alien when they were young. The alien told them that her species was dying and the four women allowed her to plant four alien eggs in their bodies so that the alien species has a chance to survive. Because the eggs need 50 years to mature, the stone was left behind to rejuvenate the women long enough to receive and deliver the child. Mrs. Markham states that she was successful while the other three failed because she and the child's father love each other. After giving birth to a human-looking baby-girl with the help of her daughter, she grows old again. However she stays alive and seems to be healed from Alzheimer's - a gift from the alien for the delivery of the baby. Dr. Markham, who can't have children, takes care of the baby, which is her half-sister (or 'third'-sister when the alien egg is taken into account).