Star Trek: The Next Generation - Yesterday's Enterprise

 

Season three, episode 15. Aired Feb. 19, 1990. This is one of the best episodes of the whole series. Two Enterprises, two crews, altered timelines, only Guinan notices something wrong.

https://en.wikipedia.org/wiki/Yesterday%27s_Enterprise

https://www.avclub.com/star-trek-the-next-generation-yesterdays-enterprise-1798166000

https://memory-alpha.fandom.com/wiki/Yesterday%27s_Enterprise_(episode)

https://www.denofgeek.com/tv/revisiting-star-trek-tng-yesterdays-enterprise/