WARNING: This thread WILL contain unhidden spoilers for this entry. Because this re-watch is in a slightly-subjective chronological order and not in production order, please refrain from talking about the content of other episodes or movies in this thread. If you do, please put that content inside spoiler tags. Some people here may be watching Star Trek for the first time.
#38: Star Trek: The Original Series, season 2, episode 4 "Mirror, Mirror"
Written by Jerome Bixby, directed by Marc Daniels.
Stardate unknown (December 2267)
(Some events take place in the Mirror Universe)
"In every revolution, there is one man with a vision." - Captain James Kirk
Captain James Kirk, Commander Scott, Doctor McCoy, and Lieutenant Uhura are on the Halken homeworld negotiating for dilithium mining rights. The Halkens absolutely refuse to allow it. The leader of the Halkin Council, Tharn (Vic Perrin) cites the possibility that, while the Federation is a peaceful organization now, they do not know how they might use the dilithium in the future. Dejected, but not defeated, Kirk insists on further talk in spite of the Halkins' unyielding code of ethics and the landing party returns to the ship due to a building ion storm. While beaming aboard, the ion storm causes interference and the landing party's patterns seem to disappear. Instead, they step off the transporter and onto an Enterprise that is the same... but different.
Originally released: 8 October 1967
"Mirror, Mirror" on Memory Alpha
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Visual effects comparison of the original broadcast & the Remastered versions.
https://www.youtube.com/watch?v=wgIXCZlTWy0