So many episodes from the first season of Deep Space Nine have been lost in a void that it takes almost an act of will to pierce the fog and rediscover what exactly was accomplished in the formative development of the series.
"Past Prologue" is a prime example. It will never be mistaken for an entry from later seasons, even the first half of the second, for which it could be mistaken, yet it's one of the most important and necessary entries the creators could have contributed. You'll be astonished, once you hear what's in this one, because I doubt for most fans, even of the series, will be able to identify it by title alone (hopefully the Fan Companion can start to rehabilitate its reputation). The funny thing is, if this had been an episode of Next Generation, this wouldn't be a problem, but that's what Deep Space Nine essentially was, a whole series of episodes that would've been highlights for any other Star Trek. It was harder to appreciate this early on.
Okay, so the first thing you need to know is that it's the first appearance of Garak. This is notable in its own right, even before you're reminded that he isn't seen again until the next season, and grows to mythic status at the end of that one. But this one's just as notable as "The Wire," not only introducing the "plain, simple tailor," but his ideal relationship with the originally, incredibly naive Dr. Bashir. Those fans who only became interested in the series later on are missing out on a version of Bashir that's one of the most interesting characters in any version of the franchise, a brilliant but inexperienced, wide-eyed dreamer who is anything but ready for the DS9 experience, and is therefore all the more remarkable for surviving it entirely intact (in the third season, you begin to appreciate his rough push into maturity).
Garak could easily have been a series regular, and nearly was in later seasons, but it's worth noting that as far as anyone knew just from viewing this first season, he was a one-off character who played like no one else since Q as guest characters go. That's reason enough to give "Past Prologue" another shot, yes?
There's also the Duras Sisters, unofficial MVPs of Klingon guest stars, arguably more significant than Gowron, only eventually upstaged by DS9's own Martok. They were mainstays in Next Generation, and were blown up in Generations, and were one of several attempts by the show's creators to maintain a link from one series to another, and as always, they're worth the visit, no matter where they are.
The crux of the episode isn't even any of this! It's actually the first hard look at the pathetic state of Bajoran society, and the terrorist mentality that pervades it after decades under Cardassian Occupation. It's not really an essential Kira episode (there are two others from this season that more easily lay claim to that title) so much as a necessary look at the Bajorans, the whole reason Starfleet took on this assignment.
Like a direct answer to the pilot, "Past Prologue" is a look at the grim everyday realities of a space station, and sets up what most of the rest of the season couldn't really live up to.
franchise * series * essential * character
Memory Alpha summary.