One of the episodes long recognized as a classic is this unusual revisit of a prior series regular (though I've listed the similar experience of Kes returning to Voyager in "Fury" as another classic, this is far from a typical Star Trek fan's view) as an alternate reality allows the late Tasha Yar to stage a comeback, although she goes back and forth during the episode as to whether or not this is a good thing.
"Yesterday's Enterprise" references in its title one of the many nifty aspects of the episode. The lineage of the name Enterprise as a ship in Starfleet was established by Pike and Kirk's commands in the original series as well as the next one under Kirk in the films. Picard's command of the Enterprise-D was the next one established in continuity, and then the Enterprise-C in this one. It wouldn't be until Star Trek Generations that we learn anything about the Enterprise-B, while the NX version was established under Archer in Enterprise. Of course Picard also commands the Enterprise-E in the movies.
Anyway, we're chiefly concerned with the Enterprise-C for the purposes of this episode. After Kirk's heydey and before Picard's there was considerable unrest between Klingons and Romulans. Worf's backstory involves some of the particulars, although his presence isn't really relevant to "Yesterday's Enterprise." The ship in question becomes time-lost in Picard's present in a crucial incident, which alters the timeline so that Starfleet has become more militaristic, but more important in this reality Yar is still alive (after all, she died pretty randomly in the more familiar one, in "Skin of Evil," which I've also listed as a classic).
She proves useful in bringing the older crew up to speed, complicated when Captain Rachel Garrett doesn't survive the experience that brought her ship into the future. An odd kind of bonus is that Yar starts to fall for the de facto new commander of the ship. This becomes relevant the longer she becomes aware that as far as Guinan is concerned, she shouldn't exist.
That's the most interesting aspect of the episode for me. Guinan was often kind of overly mysterious. The creators kept hinting about her backstory and abilities (Generations eventually revealed exactly what species she was and how she ended up hanging out with Starfleet), and sometimes that made for some incredibly interesting material, even though no single episode was ever based entirely around her (strangely enough). This one came closest. She's the only member of Picard's crew that is aware that reality has been altered, and she lets Yar know that she's the most obvious blip. This might be considered incredibly insensitive. If Guinan had remained quiet about that Yar wouldn't have made the decision she does at the end of the episode, and we wouldn't have Sela later in the series (also played by Denise Crosby).
But that decision is its own kind of redemption for the character, alternate version or not. She goes out much more heroically, choosing to lend her expertise to the Enterprise-C crew when it goes back in time and undoes its undoing of the timeline (does the doing of the timeline?). Anyway, feel-good moment for everyone, except when you realize the crew is headed to its own doom, and that Yar will become a Romulan prisoner, and mother of a real Romulan watchyourmouth.
At this point in Star Trek lore, acknowledging any kind of continuity was still a rarity, so something like this would have been special one way or another, but it's still an excellent and unexpected episode. And yes, technically there was a female captain of an Enterprise!
franchise * series * essential * character
Memory Alpha summary.