"Day of the Dove" by Jerome Bixby
This has always been one of my favorite episodes. Yes, I would listen to Michael Ansara read a technical manual.
This one throws you right in the action! Kirk, Chekov, McCoy, and a redshirt beam down to a planet. They've received a distress call, but there's no sign of a colony every having been there. Klingons beam down and capture them. Apparently, they also got a distress call and their ship has been horribly damaged. Chekov attacks, claiming Klingons killed his brother. Kang, the leader, tortures Chekov until Kirk agrees to beam them all up to Enterprise. However, he signaled Spock so the landing party beams up first. They Klingons are taken prisoner. They beam over the remaining Klingons from their ship, including Kang's wife and science officer, Mara. Then they destroy to damaged Klingon ship.
Nobody notices the spinning ball of light.
Uhura can't contact the Federation and the ship starts moving at Warp 9 towards the edge of the galaxy. "The entity then traps 392 members of the Enterprise's crew below decks by closing bulkheads and making them impenetrable. The 38 remaining members of the crew are equal in numbers to the Klingons. With tempers high – and spurred on by the sudden materialization of swords and other antique hand weapons – they begin to fight."
We find out from Sulu that Chekov is an only child. Tensions - and racism are high. Everyone is effected eventually, even Spock. Chekov attempts to sexually assault Mara at one point and it's a disturbing scene. I imagine as a child I didn't get the full horror of it.
Spock figures out there's an alien aboard manipulating them all. It appears to "feed" on hostility and war. Dying crew are suddenly fine. Kirk realizes the entity means for them to be fighting each other forever. He convinces Mara it isn't a trick and they use (dangerous) intraship beaming to get to Kang in Engineering. Kirk and Kang fight, but eventually Kirk tosses his weapon and gets Kang to see the entity and what it's doing to them. I have always loved the ending of laughing the damn thing off the ship.
A really good episode with a lot of tension and danger. I loved how Mara is afraid of being taken prisoner and her comments about the Klingons needing to expand to survive. It was good to see their point of view (even if it's propaganda). Ansara is terrific as Kang and Chekov, McCoy, and Scotty all get some nice scenes.