the story: The crew takes the Defiant into enemy space, only to find the enemy aboard the ship in the form of a Founder who, as a shape-shifter, could be anyone...
what it's all about: Fans tend to forget how awesome "The Adversary" is, because in a lot of ways it was rephrased and blown to even more epic proportions in the fourth season premiere, "The Way of the Warrior," which has the distinctions of not only bringing in the Klingons, but introducing Worf into the series. By contrast, "Adversary" actually features a one-off alien threat that's not even seen.
But it creates its own history, quite decisively, too. The first and most obvious way is in Sisko promotion to captain. It's the culmination of everything the season accomplishes for Sisko, rebuilding him into the strong center of Deep Space Nine in every way possible. As the only lead character (until Star Trek: Discovery some twenty years later) of a Star Trek series to not be a captain in the pilot episode, Sisko always seemed to be at a disadvantage. Well, by the end of the third season, not only does he run a station, but his own warship, and so of course he finally gets the big promotion.
But that's really only in the running for most interesting thing about the episode, because this is the one that really ramps up the threat of the Dominion. Curiously, after the second season ended with "The Jem'Hadar" and this one began with "The Search," the Dominion had all but fallen by the wayside, lost to a number of other intrigues that only sometimes intersected with it. This was a season of making sure fans understood what the stakes were. So by the end of it, the threat of the Dominion can be revisited, and vastly intensified, as the nature of the Founders becomes chillingly apparent for the first time. These are villains who will stop at nothing. The fourth season's "Homefront"/"Paradise Lost" would pick up on the paranoia aspect, but most importantly, Odo is forced to do something that would later have a cataclysmic impact on his life, and all because of this Founder oath: "No changeling has ever harmed another."
Which of course means Odo is forced to kill one. We've already seen how conflicted he is over the revelation that his long-sought-over people run the Dominion. To then be forced into a situation where his outsider status is made about as stark as it can be...It's high drama. It's matched by the first real sustained action-oriented drama of the series, which would of course later feature two final seasons entrenched in the Dominion War, making "The Adversary" the first true preview of what was to come.
That makes it hugely significant, and another chance for more hesitant fans to catch a glimpse of what all the fuss was about. That makes it a classic.
- franchise - To that point , really on "The Best of Both Worlds" had given fans a glimpse of this kind of storytelling in Star Trek.
- series - But it's just a glimpse of what would become Deep Space Nine's hallmark.
- character - Sisko's promotion, Odo's fateful moment...
- essential - It's a classic, okay?
Ken Marshall (Eddington)