The straight up: Some old, forgotten bit of biological warfare has come back to haunt the station, resulting in something called the "aphasia virus" that results in, well, aphasia. Oh, and eventually the neural pathways in the brain break down and kills the person. One by one the crew falls until everyone is affected except Quark and Odo. What is at first thought to be a bit of sabotage left behind by the Cardassians turns out to be a booby trap built into the replicators during the station's construction by the Bajoran underground.
Obviously, they found a cure before the end of the episode otherwise there'd be no show. This is a bit of an issue. Not that a cure was found before the end of the episode: just that it was found so quickly and easily and that much of that was handwaved by the writers.
It's a nice little morality play in how biological warfare has a nasty habit of biting everyone in the ass, including the people who created it (Bajorans are affected as well as everyone else).
Again, this is a case where there's some really clever execution. The virus is released into the general population via the replicated food, which means everyone who's eaten anything is down and out. The fact that some people are affected more quickly than other individuals in the same species is easily explained with zero technobable (the virus is adaptive so it customizes the attack on each host). The nature of the virus and what it does actually sounds like it could come from a regular medical drama (well...a non-science fiction one at any rate). And Kira's hunt for the virus's creator is also logically and methodically carried out.
Why this episode doesn't live up to its premise, I don't know. I was mildly entertained by it (especially Kira's kidnapping of the one man who could find a cure for it), but I wasn't terribly engaged or all that worried about the fate of the crew.
Some stand-out points from this one:
- O'Brien's background war against the technology on the station once more takes center stage. There've already been one or two bits at this point where O'Brien has to fight the station computers. This shows the other half of his job: rectifying the long-term neglect and shoddy maintenance the station had under the Cardassians. (Remember: it was just used for ore processing before it became the gateway to the Gamma Quadrant.)
- Sisko being a little bit of an asshole at the beginning of the episode. O'Brien's been running around on almost no sleep overseeing the engineering crews as they fix and restore vital functions around the station. When Sisko gets a bad cup of coffee out of the replicator, he yanks O'Brien off all his other duties and makes him fix the replicators. Not all the replicators, mind. Just the ones on the command deck and in the crew quarters.
- The Sisko-Jake father-son relationship. You really do get the sense that Sisko is dead worried when Jake becomes infected. Heck, Sisko even takes time out while things are falling to crap around him to go check up on his son. The chemistry between Avery Books and Cirroc Lofton is palpable as the two characters physically and verbally interact the same affectionate way a father and son interact.
- The fact that the background Bajoran characters are running around far longer than the other races before they succumb to the virus.
- Kira's straightforward, no nonsense, no frills kidnapping of the only Bajoran who can find a cure. It's a lot like her: blunt and takes no prisoners.
- Possibly some evidence that some of the Bajoran resistance cells weren't all that concerned if innocent Bajorans were hurt or killed, just so long as they could kill a lot of Cardassians with their schemes.
- The fact that Sisko and Dr. Bashir held out the longest of the humans before succumbing. Unfortunately, I can't tell you why that's interesting (spoilers!), except to say that future revelations about these two characters make for a kind of interesting reverse-retcon on why this is so.
- It's mentioned in passing that Odo figured out that Quark told a lie because he said that Rom fixed the replicators in the bar. Odo notes that Rom "is an idiot who couldn't be trusted to fix a straw," a sentiment with which Quark agrees. Considering that Rom really is an engineering idiot savant (a fact that takes Quark by complete surprise when it's revealed), the conversation is kind of funny in retrospect.
- That the Ferengi have immune systems made of steel. Nothing gets these guys sick.
- We find out that before opening his bar, Quark served on a Ferengi freighter for 8 years.
Overall, not a great episode or one that's particularly memorable. But it's certainly not the worst episode DS9 (or any other Star Trek) has ever done.