the story: Jadzia discovers the existence of a hidden, unstable host of the Dax symbiont.
what it's all about: The idea of the joined Trill species originated in Next Generation's "The Host," which surprisingly in retrospect looks more like a bizarre case study in Star Trek intolerance than a preview of a major element of Deep Space Nine, so it's good that Jadzia and later Ezri Dax had so much time to explore the nuances of the concept in greater detail. "Equilibrium" introduces a major wrinkle in the concept and also gives us a rare glimpse into Trill culture, which aside from Enterprise's Denobulans still stands as the most relatively unexplored aliens of the whole franchise despite plenty of exposure. It would be like if Star Trek had stuck to Klingons as the enemy. It sounds kind of inconceivable when put into words.
Granted, as "Equilibrium" makes pretty clear, being chosen to join is a big deal among Trill. So much so (and this is better explored later in the season with "Facets," by the way) that we meet a renegade recipient who wasn't at all qualified for the honor who nonetheless hosted Dax for a short time, and was subsequently suppressed with the collective Dax memories, until he begins to resurface, unconveniently, for Jadzia.
That's actually how the episode falters. This is weighty material, but much of it is buried under symbolism (the recurring image of masks) and admittedly catchy music, both of which make the intended bad guy seem a lot more sympathetic than either of his subsequent appearances (in the seventh season, "Field of Fire," plus a ridiculously creepy cameo in "Facets"). I get that this was a complicated subject, and that Deep Space Nine was known for nuance, but it just wasn't very successful this time.
As a soft exploration of Trill life, it's a welcome experience. But it kind of degenerates into something that could just as easily not featured Trill life at all, even with the fun visit to the symbiont caves at the end of the episode.
franchise- This one's weird, it's kind of series specific while at the same time... series- Not particularly adamant about it, either.
- character - But it's a good one for Dax.
- essential - And kind of important to the overall Trill portrait presented in the series.