In this episode, Dr. Bashir and Jadzia Dax travel to a planet that was attacked by the Jem'Hadar 200 years earlier. In order to make an example of its people, the Jem'Hadar infected them with an incurable disease called the "blight" that leaves spidery black lesions on the body that at a random time (usually before adulthood) will "quicken" into a spreading red form that results in a slow, painful death.
Bashir is distraught when he learns that the closest equivalent to a doctor these people now possess is Trevean, a man who offers a swift and comfortable death through herbal treatments to the recently quickened. Bashir attempts to develop a cure with the help of recently quickened volunteers, but fails when he finds that the electric fields from his equipment are causing the virus to mutate rapidly.
When the others leave to return to Deep Space Nine, Bashir elects to remain behind to help a recently quickened woman who is in late pregnancy. While he administers his anti-viral treatment to her, it appears to have no effect, and she dies shortly after childbirth. However, the child is born without the lesions, as the treatment apparently acts as a vaccine rather than a cure. This gives hope to the people of the planet that by applying the anti-viral treatment to pregnant women, the next generation can be free of the blight.
Back at DS9, Bashir is seen still working on a cure for the disease in those who already have it, but is unsuccessful. Sisko reassures him that at least the next generation will be free of the disease, but Bashir does not seem entirely satisfied.