The hot streak known as the final episode of the series begins!
Rarely does any episode of any series, Star Trek or otherwise, beg to have more episodes written about the same story. I'm not talking about an arc that will by definition have more stories attached, but an episode like, say, "Mirror, Mirror," the original series episode that famously introduced the Mirror Universe, the alternate reality where everything is flip-flopped. Deep Space Nine eventually took that ball and ran with it for five additional adventures.
The Mirror Duology brings us back. Except it does so in such an intriguing way that it essentially duplicates the "Mirror, Mirror" effect in more ways than the obvious. It's fair to say that how the Mirror Universe happened in the first place is at least as intriguing a topic as how Klingons lost their ridges (answered in the preceding Klingon Duology). Enterprise decided on something extremely clever, depicted in the opening moments of this episode.
Remember First Contact, in which we learn that Vulcan's were humanity's initial encounter with aliens? In this version of those events (borrowing just enough actual footage from the movie to make it work), Cochrane extends a gun rather than his hand, thus establishing an entirely different relationship between the two species than the one we've known. That leads to the Empire, rather than the Federation. (And to think Archer spent a long time resenting Vulcans for not letting humans take whatever they wanted!)
The rest of the episode features the familiar crew, plus a welcome return from Forrest, killed off in his regular life earlier in the season. The crux of the story revolves around discovery of the original series Defiant, subject of the episode "The Tholian Web," the presence of which drastically affects the balance of power, or so Evil Archer would love to believe. A lot of it harks back to "Mirror, Mirror" itself, especially in this installment.
But it is absolutely essential.
franchise * series * essential * character
Memory Alpha summary.