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.
#20: Star Trek: The Original Series, season 1, episode 18 "Arena"
Written by Gene L. Coon & Fredric Brown, directed by Joseph Pevney.
Stardates 3045.6 through 3046.2 (September 2267)
"We will resolve your conflict in the way most suited to your limited mentalities." - The Metrons
Captain Kirk takes a landing party down to the Federation colony of Cestus III with the expectation of relaxing. They find the entire colony has been destroyed by an unknown force and soon both the landing party and the Enterprise come under fire. After a tense moment, Kirk on the ground and Sulu on the bridge of the Enterprise manage to push back their attackers. Once back aboard, Kirk orders pursuit of the enemy vessel with visible anger and at great risk to the ship's engines. When they do catch up to the enemy vessel, is has suddenly stopped and is without power. The Enterprise herself is also ground to a halt and held in place by an unknown force. Both ships have entered the solar system of a highly advanced race that despises violence. As punishment, both ship's Captains are made to fight to the death on a barren planet.
Originally released: 19 January 1967
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What did you think?

Visual effects comparison of the original broadcast & the Remastered versions.
https://www.youtube.com/watch?v=tdvEhx2nh1Q