The teaser opens in Ten Forward where Guinan gives Worf a drink. After being very impressed with the beverage, Guinan tells him that it's Prune Juice. While this scene can be regarded as a mere one shot comic relief moment, Worf would continue to order Prune Juice throughout both TNG and DS9. It's such a nice genuine "origin" moment that it's almost hard to believe it comes from this episode because not only is it not a story about Worf, the episode wishes him away until the very last minute..... and for a very good reason. Welcome to the episode of Star Trek: The Next Generation where quality television (not just Star Trek) hit a high mark.
The origins behind the creation of "Yesterday's Enterprise" came about from two different story ideas that were merged together. A story involving the Enterprise C and a story that would give Tasha Yar a better send off. It's awkward when you look at these two details and think "How the heck would these two go together?". Well, having Ron D. Moore certainly helped since this is the same guy who would take very strange elements from genuinely bad episode of the first two seasons and turn it into the best character driven episodes of TNG. What follows has got to be the biggest "WT*?" moment that the producers thought would be suicidal. You want to have this great concept focus on a bunch of characters who we don't even know and probably don't like? And I'm not talking about the Enterprise C crew, I'm talking about the show's regular actors playing different versions of their characters. Not a "mirror" evil version, just the same person who had gone through a different set of circumstances.
So the Federation/Klingon timeline was created. The differences between the real timeline and the altered timeline is one of this episode's highlights. Captain's log is now military log, Stardate is now combat date, the Enterprise is now referred to as a Battleship, the uniforms have been slightly tweaked and the bridge has been massively changed into a much darker and colder environment that even includes more tactical stations. Even Ten Forward feels more like a middle school cafeteria than a nice quiet lounge, with crowd chatter drowning everything out and announcements playing over the speaker on a constant basis. The character changes are also subtle, like how Picard refers to Riker as commander instead of Number One.
But the absolute reason that this episode exists is of course Tasha Yar. When it came to her original departure, everyone felt that it was an incredibly weak point to the whole show. What we get here is an in-universe acknowledgement of that events with Guinan telling Tasha that it was an "empty death. A death without purpose". Yep. This writing team took an episode where the higher ups were all too happy to just be rid of Tasha Yar and say "I did a good job!" and tell it "No, it was crap and we're going to acknowledge it as such". And what a great way to motivate a character as well. Realizing that in a time of war and endless battles, you are at the hight of your game where as in a time of peace and prosperity, you die in the most pointless ways imaginable. What can you do? Well, if you're Tasha Yar, you don't stand around and wait for yourself to disappear, you go out fighting. The scene where she requests a transfer from Picard is one of the saddest moments in Star Trek and it gets me every time. Tasha doesn't want to just sit by and wait for her to die out of existence, and Picard doesn't want to send her off to her death even when he realizes that death will be certain no matter what. If she can give the Enterprise C just a few more seconds, that may be enough to change history back.
And than we have the final battle of the Klingon/Federation war. This whole scenario is just so awkward. We have the Enterprise D sacrificing herself to the Klingons to ensure the Enterprise C sacrifices herself to help the Klingons to prevent stop a war. And just before we see the Enterprise D get completely destroyed, the Enterprise C enters the rift and everything is back to normal. Worf is at his station, Troi is there and Guinan is relieved that not only are things back to the way they were, but she has someone to thank for it.
A classic Star Trek episode. Everything that can be used to describe "great Star Trek" can be found in this episode. It's hard core science fiction, the characters are fantastic and the resolution and the ending spectacular. Some would argue that this ending qualifies as a reset switch, there are still things that are left that shows the events of Yesterday's Enterprise will not be forgotten. Great story telling, great action, great characters, and again, great science fiction.
I know a lot of people love "Let's make sure that history never forgets the name Enterprise" line, but I was never a fan of it. Instead, I'm going to leave a quote from SFDebris' review that brings up a point that I'm certain no one on the production even contemplated when writing this episode.