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.
#31: Star Trek: The Original Series, season 1, episode 29 "Operation -- Annihilate!"
Written by Steven W. Carabatsos, directed by Herschel Daugherty.
Stardates 3287.2 through 3289.8 (November 2267)
"I did it! It's finally gone! I'm free!" - Denevan pilot
As the Enterprise nears the planet Deneva, they are having trouble establishing communications, making Captain Kirk visibly worried. Commander Spock makes Kirk aware of an alarming pattern of outbreaks of mass hysteria along a single trajectory across the galaxy, pointing out the Deneva would be the next planet this so-called wave of hysteria would hit. The Enterprise reaches Deneva just in time to watch a Denevan deliberately fly their small craft into the planet's sun before they could do anything to stop it. Kirk, now deeply concerned, orbits Deneva and beams down personally with a team to find out what is going on. In a city of over 100,000 people, not a single person can be seen outside. The situation becomes stranger when a group of men wielding blunt instruments appears and charges while also warning the away team to get away from them.
Originally released: 13 April 1967
"Operation -- Annihilate!" on Memory Alpha
THE STAR TREK FRANCHISE REWATCH PARTY MASTER LIST
Check here to find out where you can stream or digitally purchase Star Trek: The Original Series in your country. Star Trek: The Original Series is also available on DVD & BluRay.
What did you think?

Visual effects comparison of the original broadcast & the Remastered versions.
https://www.youtube.com/watch?v=REGtTGAUeC8