[TNG] Season 3, Episode 15: Yesterday’s Enterprise
This episode has a pretty perfect blend of character moments, suspense, and world-building. It also gives Tasha Yar a more noble send-off. A lot of the ideas in this episode will later be run into the ground by both Trek and science fiction in general, but “Yesterday’s Enterprise” takes advantage of the alternate reality premise, showing how these characters are shaped by the different events of the new timeline in a believable and insightful way.
Read more at Memory Alpha
Notes:

Guinan says they have to send the ship back through the portal, even though they will die. They were paramount in the peace treaty between the Klingons and the Federation

Yar gets funny looks from Guinan. Guinan tells her that she’s supposed to have died in a mediocre episode

Yar decides to go back with the C crew. “Sure, it’s only super important that everything happens the way it did. Throwing a tactical officer from the future into the mix should be ok. Watch out for Romulans!”
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