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.
#57: Star Trek: The Original Series, season 2, episode 21 "Patterns of Force"
Written by John Meredyth Lucas, directed by Vincent McEveety.
Stardate unknown (2268)
"A man who holds that much power, even with the best intentions, just can't resist the urge to play god." - Doctor McCoy
The Federation has lost contact with Professor John Gill (David Brian), a cultural observer who was studying the Ekosian people of Ekos in a system they shared with the Zeon. The Enterprise is sent to reestablish contact with him. Upon arrival in the system, they are met with a nuclear missile launched at the ship from Ekos. But Ekos isn't suppose to have technology that advanced. Captain Kirk and Commander Spock beam down to the planet in disguise and are immediately met with the sight of a Zeon man being chased and beaten by a group of Ekosians dressed exactly as Nazi officers from Earth's 20th century. Seeing a news reel of how the Ekosians have unified the entire planet under a mission of an ethnic cleansing of all Zeons on Ekos and on their own homeworld, and how John Gill himself has been elevated to the position of the Fuhrer of this version of the Nazi Party, Kirk desperately tries to find a way to make contact with Gill to find out why he violated the Prime Directive and chose to recreate one of the darkest pages in Earth's history.
Originally released: 16 February 1968
"Patterns of Force" on Memory Alpha (mirror)
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And with that, we're chronologically done with season 2. Onward to season 3, The Animated Series, and (if you're game) Star Trek Continues to round out the original five-year mission.