Doctor Sara Kingsley, age 35, was the head physician at the Darwin Genetic Research Station in 2365. She led an experiment to produce "superhuman" children who would be immune to virtually all forms of disease. During the experiment, the children's advanced and proactive immune system, which sent out airborne antibodies to destroy potential infections before they entered the body, interacted unexpectedly with an alien virus and became dangerous to normal humans, mimicking the effects of rapid aging. The whole of the station became infected with the disease.
Kingsley asked the USS Enterprise-D to save the children, who had been cryogenically frozen, believing they were the future of the Federation. Unbeknownst to Kingsley the children were the cause of the disease, and Katherine Pulaski ended up infected as well. Jean-Luc Picard and the Enterprise crew were able to save Pulaski using a special manipulation of the transporter – a technique they next intended to use with Kingsley and the crew of Darwin Station.
After being cured by the transporter, Dr. Kingsley and her associates continue the research on the children with the hope that they would leave quarantine and join the human society someday. (TNG: "Unnatural Selection")
The old and aging Kingsley was played by actress Patricia Smith; her true younger and healthier appearance was never seen. In an early draft of the script, Kingsley's name was established as "Sara Mandel."
In an earlier concept of the story, a strikingly beautiful and fully recovered Dr. Mendel would be seen on the viewscreen of the Enterprise during the last minutes, thanking the crew for saving the lives of everyone on the station. To save time and money (with a younger actress) this idea of a scene was discarded and the audience never gets to see Kingsley in her true, young and healthy form.