While on the mission in the Delta Quadrant, they receive an automated distress signal from a planet on the edge of Dominion space. When they beam down to the planet to investigate, they find a ruined city and people with bluish lesions on their faces. They then encounter a woman with red lesions who collapses in agony in front of them. As she dies, a man explains to Dax and Bashir that this civilization is infected with something known as "the blight". The Starfleet officers are taken to a clinic where a local doctor, Trevean, explains the origins of the blight. This civilization, referred to as the Teplans, were once a very advanced civilization. But when the Dominion were expanding to the area and attempted to subjugate them, the Teplans attempted to resist. It ended poorly: not only did the Jem'Hadar destroy most of the major population centres, the Dominion infected the planet with a virus which is now known as the Blight. It progressed so rapidly and infected everyone to the point that all Teplans now have it, even from birth as the virus is transmitted in-utero. Born with the blue lesions on their faces, when these lesions turn red, it means that the disease has "quickened" in them meaning that they have reached the stage where the disease will ultimately kill them in days. Trevean is revealed to be a palliative-care doctor of sorts: when people quicken, Trevean administers a blend of herbs that quickly kill those who have quickened so that they do not suffer the horrible agony that is prevalent in the disease's terminal stage. The distress signal that Dax had discovered is from an automated beacon from 200 years ago, from when the Dominion first invaded.
Kira has been monitoring a Jem'Hadar patrol in the area and notes that they need to leave as soon as possible. However, Bashir feels that he can do something to help the Teplan people. To that end, Kira agrees to hide in a nearby nebula for a week, while Dax and Bashir try to see if they can do something for the Teplans. With the help of a local pregnant woman named Ekoria, Bashir sets up shop at her residence and sets out to do his work. They immediately isolate the virus that causes the Blight and Bashir synthesizes an antigen that he uses on a group of volunteers eager to see Bashir at work, including Ekoria. Trevean warns Bashir of prior people who have promised cures to Teplans only to deliver nothing. Bashir refuses to promise anything but Trevean warns the group of volunteers that they will require Trevean's services soon enough.
Trevean soon proves to be correct: the group of Teplans start to suffer horribly while lesions develop suddenly in front of Bashir's eyes while during routine scans. Bashir realizes that the electromagnetic fields from his instruments are allowing the disease to progress rapidly and it dawns on both Dax and Bashir that not only the disease was designed to be fatal, but it forced the Teplan civilization to abandon technology just to survive. Now aware of the group of dying volunteers, Trevean administers toxic cocktails to all affected patients to stop their suffering. Only Ekoria remains of the group of volunteers, the rest now dead.
Bashir is despondent: he was so confident that he would find a cure so quickly but didn't realize that the Dominion had engineered the virus in this fashion to make it so difficult to even eliminate, much less treat. Just as he is about to give up, Dax tells him that just because Bashir couldn't find a cure, does not mean that the Blight is incurable. When Kira returns to the planet, Bashir resolves to stay behind and will send a signal to Deep Space Nine when he is ready to be picked up.
Using primitive tools available, Bashir resolves to care for his only remaining patient: the pregnant Ekoria, who is now in the initial stages of Quickening. He resolves to help her to at least ensure that her child is born. Trevean meanwhile wishes for Ekoria to seek his "treatment", noting that her child is likely infected with the blight in-utero and that death would be preferrable to living in suffering, but Ekoria refuses. A few weeks later, Bashir induces labor on Ekoria and she delivers a baby boy without any of the blue lesions of the Blight. Bashir realizes that the antigen he has created is actually a vaccine that helped Ekoria's son fight off the Blight. Ekoria, finally realizing that her son will not suffer from the disease, dies in exhaustion.
Trevean and his group of healers are astonished as to what Bashir was able to accomplish. Bashir agrees to help Trevean to manufacture the vaccine and Trevean himself makes it his task to ensure the vaccine is administered to every pregnant woman on the planet. The Teplans begin to celebrate the first child born free of the Blight but Bashir stands away from the Teplans, not feeling like celebrating. Back on the station, Sisko gives Bashir his commendations for helping the Teplan people, but Bashir does not feel the accolades are deserved. The next generation of Teplans may be able to be born free of the Blight, but the people still suffering will continue to die. As the episode ends, Bashir runs his computer through simulations of the virus's progression trying to see if there is something he missed in his quest to help the Teplans fight the horrible disease.
- Artistic License Medicine: It's extremely premature to call Bashir's antigen a successful vaccine. For one thing, this vaccine was only confirmed to be successful in one person, and while it worked for Ekoria's son, there wouldn't be any guarantee that it would work with other pregnant women, especially those who have progressed later in term than Ekoria has. In addition, a virus like the Blight already existing in the mother's body would likely infect the fetus during the early stages of pregnancy, yet a vaccine administered in the late stages of pregnancy appear to be successful. The episode seems to reference this at the end when Bashir is running simulations on his computer to find out exactly what happened.
- Bittersweet Ending: Try as he might, Bashir cannot find a cure for those presently suffering from the blight. However, he does accidentally create a vaccine to prevent babies from being born with it if administered to pregnant women, thus saving the next generation (and those in the future).
- Break the Haughty: Bashir goes down to the planet expecting to have the blight fully cured in just a few days. Boy, is he wrong. By the end of the episode when the first baby born free of the blight is presented to the populace, Bashir wants nothing to do with the celebration, and prefers to stand off to the side. Later, when Sisko is offering his congratulations, Bashir sharply retorts that he doesn't deserve any.
- Cold-Blooded Torture: Implied to be the fate of those who have falsely promised help to those dying of the blight.
- Cruel and Unusual Death: The whole point of the blight.
- Cruel Mercy: This planet resisted the Dominion, and as punishment the Jem'Hadar infected them with blight that they couldn't cure, had a hundred percent infection and mortality rate, and will eventually kill the victims at a random point in their life. To top it all off, the virus is made worse in the presence of electromagnetic fields, meaning that the Teplan civilization had to renounce technology and revert to medieval standards to just survive. Thus, instead of simply killing their enemies outright like they usually do, they condemned the populace to die a slow death.
- Death by Childbirth: Sort of. Ekoria's already dying of the blight, but giving birth finishes her off.
- Determinator: Ekoria, now in the stages of Quickening, is determined to survive until her child is born, even refusing Trevean's "treatment", and enduring suffering up until the birth of her son and her death.
- Dystopia: We don't get a name for the planet itself, but it's sure as hell this trope thanks to the blight.
- Earworm: "Come to Quark's! Quark's is fun! Come right now! Don't walk, run!"
- Face Death with Dignity: Trevean's clinic euthanizes those who have quickened and ensures they are as comfortable as possible.
- Heroic BSoD: Bashir goes into one when his attempts to help only make things worse. He comes out of it when they finally yield some measure of success.
- Hope Spot: It looks like Bashir is making excellent progress finding a cure for the blight...and then things really go to hell.
- Human Aliens: Outwardly at least, the Teplan inhabitants look identical to humans (or would, if it weren't for the lesions from the blight), although their physiology is very different to either humans or Trills, meaning the disease can't harm Bashir or Dax.
- Implied Death Threat: Kira delivers one to Quark:"If all your little advertisements aren't purged from our systems by the time I get back from the Gamma Quadrant, I will come to Quark's, and believe me, I will have fun."
- Kick the Dog: Simply because the people of this planet resisted the Dominion 200 years ago, the Jem'Hadar made an example of them, by infecting them with a "blight" which they are born with and which "quickens" itself randomly in a person's life, creating red lesions when it becomes active, and killing them shortly after in particularly painful fashion. That'd be bad enough, but it turns out the electromagnetic fields from medical instruments (as Bashir learns the hard way) amplifies the virus, and causes it to mutate. These people must've really, really pissed off the Dominion.
- Layman's Terms: Dax translates Bashir's Technobabble for Ekoria.
- Mandatory Line: Sisko doesn't appear until the final scene. Worf, O'Brien, Quark, and Odo only get a couple of lines in the teaser. note
- Meaningful Name: Trevean is an anagram of "veteran" and Ekoria is a (loose) anagram of "rookie." The writer, Naren Shankar, said that kind of thing helps him with characterization.
- Medieval Stasis: Enforced by the blight. Electromagnetic radiation makes the disease rapidly progress, so their once advanced civilization had to regress to survive. Bashir discovers this when all his equipment makes the disease even worse and he has to go low-tech to continue his work.
- Mercy Kill: Trevean's actual job. Bashir initially mistakes him for a doctor.
- Mood Whiplash: The cold open is Kira, Odo, and Worf acting as though Quark sneaking an annoying advert into the ship's systems is a crime equivalent to high treason.note This hilarious introduction soon segues into a story about... the protracted genocide of a planet across generations and Julian's largely futile efforts to put an end to it.
- Nice Job Breaking It, Hero!: Bashir's medical equipment only makes the disease worse.
- Paranoia Fuel: In-Universe—nobody knows when they'll quicken.
- Pregnant Badass: Ekoria. Let's face it, she's a Determinator who resolves to fight this disease until her son can be born. While she still dies, she ultimately knows that her son will be free of the Blight.
- "Ray of Hope" Ending: Bashir fails to cure the people living with the blight, but he accidentally figures out how to immunize their unborn children.
- Surprisingly Realistic Outcome: This episode is a Deconstruction of a common plot in Star Trek—a long-standing medical problem is cured by the miracle of Starfleet medicine. More likely, a disease that has gone uncured for so long won't be so easily resolved—and indeed, Bashir never finds the cure he hoped to find. He still has more success than anyone before, but the full benefits of his discovery won't be felt for another generation or so.
- Wasteland Elder: Trevean, a particularly cynical example.
- Truth in Television:
- While antigens are actually part of a vaccine, the episode doesn't treat Bashir's antigen as a vaccine until the very end when it is revealed that it can be used on pregnant women. While it seems odd that antigens would be used in this fashion, the notion that a vaccine can be used on an existing infection does have some precedence as Shingles is a disease arising from a dormant infection from Chicken Pox, and we do have Shingles vaccines available today.
- The DTaP vaccine is an example of a vaccine that is administered to pregnant women to help protect their unborn children from infection from Pertussis, which is the bacteria known to cause Whooping Cough, a condition that can be fatal to newborns.
- Bashir standing off to the side as Trevean shows the first blight-free baby to the Teplan populace is in reference to Jonas Salk, the creator of the first polio vaccine. He wanted no credit in its creation and wanted nothing more than to help prevent a disease that at the time, had ravaged millions.
- What the Hell, Hero?: Dax gives a beautiful one to Bashir, telling him that it's pure arrogance to think that there isn't a cure for the Blight simply because he couldn't find it. Funnily enough, this is in the wake of Bashir chastising himself for arrogance of a different kind—thinking that he could find a cure so easily for a disease that had gone uncured for so long.
- "You!" Exclamation: Worf, when he confronts Quark over a mug of prune juice that is basically an advertisement for Quark's bar.Worf: I ordered a glass of prune juice from the replicator in the Defiant's Mess. THIS (raises a mug with Quark's advertisements printed all over it) is what it came in! (he turns it to the side, and it plays an audio version of his ad)