This is one of those episodes that is wonderful for a Trek fan. It filled in an important story piece for Trek's history and on top of that it offered a change of pace for the show.
For those not in the know, the original series of Trek was roughly 80 years before The Next Generation and in between the series, the Klingons went from being the main villians to holding a friendship with Starfleet in The Next Generation. In this episode, we see what might have happened if the Klingons weren't our friends and that a certain act led the Federation into a long and costly war.
While the modern day Enterprise is studying a spatial entity, another Starfleet ship emerges from the entity. It's the USS Enterprise from years ago. Damaged and drifting, a signifigent change occurs. The modern day Enterprise is now lit darker and is a warship serving in a military capacity. Worf and Troi are gone but Yar is still alive and Wesley is now a full Ensign.
No one on board is aware of the changes on board escept for Guinan who must convince Picard that the "real timeline" awaits them. That involves sending the older Enterprise back through the entity to it's own time to fulfill it's place in history in saving a Klingon outpost from an attack, which would mean they're death sentence but retroactively saving millions of lives.
While I love seeing the old movie style outfits that are used in this episode and different takes on the characters being in a more harsh climate, especially Picard, it was the battle scene at the end that made me like this episode the most. Equally good is Tasha Yar coming to learn that in that other world that awaits the rest of the crew, she died pointlessly and makes decides to go out in a blaze of glory with the old Enterprise crew. This even in and of itself would have another important event happen later on in the series.
Yesterday's Enterprise makes many top Trek lists and it is certainly on mine. It's got a nice space action scene at the end, it brought back Yar, things were changed but still were enough the same, and filled in a crucial gap inbetween TNA and TOS concerning the Klingons.
