Doctor Kingsley, age 35, was a physician at the Darwin Genetic Research Station in 2365. She led an experiment to produce "superhuman" children who were intended 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, leading to a fast metabolic deterioration mimicking the effects of accelerated aging. The whole of the station became infected with the disease, as well as the crew of the visiting Starship USS Lantree.
Kingsley desperately urged 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.
While her fate is never shown and her true appearance is never displayed, it is assumed that after being cured by the transporter, Dr. Kingsley and her associates continued the research on the children in the hope that they would be able to leave quarantine and join the Human society someday. (TNG: "Unnatural Selection")
The accelerated aging Dr. Kingsley seen during several deteriorating stages - her only versions ever shown on-screen - was played by Patricia Smith.
The later introduction of the concept of a ban on genetic experiments in humans during the production of DS9 creates a problem in retrospect for this episode, even though no such ban existed from the writers perspective when this was produced, which explains why no one on the Enterprise was shocked by the nature of the research, with Picard making no arrests.
In the final draft script, Kingsley's name was established as "Sara Mandel." She was called "Sara Kingsley" in the Star Trek Encyclopedia (2nd ed., p. 240) and Star Trek: The Next Generation Companion (2nd ed., p. 73).
In an earlier concept of the story, a strikingly beautiful and fully recovered Dr. Mandel 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.
Her first name, Sara, could be a take on the fact that in some Latin languages like Portuguese, "Sara" means "Cure", while the verb "Sarar" means "to cure, to heal". Since she wanted to heal future generations of any possible diseases, this may have been intended.