For several years now I've lived with people who like to watch Star Trek with lots of other people on a fairly regular basis. I've started to slowly collect episodes based on themes, mainly for my own enjoyment but also because I find the self-contained narratives of each episode to be enormously useful teaching aids. I'm putting them here as a resource for others. There's a lot of (intentional) overlap in these lists and I'm still working on a standard format for these so, apologies where necessary.  This page is updated from time to time.

Drugs and Addiction

"Symbiosis" (S01E22)- After the Enterprise rescues a freighter crew and the ship's important cargo, Captain Picard faces a difficult dilemma: uphold the Prime Directive, or save an entire civilization from exploitation?

"The Game" (S05E06)- Wesley Crusher visits the Enterprise only to see everyone behaving strangely on account of an addictive, mind-controlling game.

"Hippocratic Oath" (S04E04)- Bashir tries to help members of a Jem'Hadar unit free themselves of their addiction to the drug the Founders use to control them. Meanwhile, Worf tries to adjust to life aboard DS9.

"Rocks and Shoals" (S06E02)- Sisko's crew crash land their Jem'Hadar ship into the waters of an alien ocean on a world where a group of Jem'Hadar have also crash landed with only one vial of ketracel-white remaining. Meanwhile, on Terok Nor, Kira finds she cannot live with herself working side by side with the Dominion.

Labor

"The Cloud Minders" TOS (S03E21)- Kirk's efforts to obtain a vital mineral are complicated by terrorists striking at the beautiful cloud city Stratos and its virulent apartheid policies.

"Lower Decks" TNG (S07E15)- Junior officers speculate on the reasons for recent unusual actions taken by the command crew near the Cardassian border.

"Bar Association" DS9 (S04E16)- Quark's employees, led by Rom, form a union against Quark's unfair labor practices and promptly go on strike.

"Author, Author" VOY (S07E20)- The Doctor completes work on a holonovel that depicts the crew of Voyager in rather unflattering roles as they abuse and scorn the Emergency Medical Hologram. When the holonovel is published without his permission, the issue of The Doctor's legal rights is brought into question. Seven reconnects with her aunt.

Gender

The Right to Life and Death

Race

Let that Be Your Last Battlefield TOS S03E15: The crew of the Enterprise find themselves caught in the middle of an intractable conflict with a bizarre fugitive alien and his equally belligerent pursuer

The Abandoned DS9 S03E06: Quark finds an abandoned Jem'Hadar child, and Odo is the only person who can control him.

Far Beyond the Stars DS9 S06E13: Experiencing a vision from the Prophets, Sisko sees himself as Benny Russell, a science-fiction writer in the 1950s, who struggles with civil rights and inequality when he writes the story of Captain Benjamin Sisko, a black commander of a futuristic space station.

Faces VOY S01E14: The Vidiians capture B'Elanna Torres and split her in to two people, one fully Klingon, and one fully human.

 

The Calculus of War (Automating & assessing the risk of war)

 The "No Win" Scenario

Yesterday’s Enterprise S3E15- A temporal rift opens, and the USS Enterprise-C emerges, changing the timeline into a reality where the Federation is in a bitter war with the Klingon Empire. The only person on the USS Enterprise-D who realizes that something is not right is Guinan, and she must help return the displaced starship to its proper time, in order for the Enterprise-D crew to save themselves.

Tribunal S2E25- When Cardassians arrest Miles O'Brien for working with the Maquis, he's put on a Cardassian trial, where the verdict is known before the trial begins: guilty.

The Caretaker Parts 1 & 2 S1E01 & 02-  The newly commissioned starship Voyager and a Maquis raider are flung into the remote Delta Quadrant by a powerful entity known as the Caretaker. (Series Premiere)

Terrorist / Freedom Fighter

Journey's End S7E20 - While visiting the Enterprise, Wesley Crusher protests the removal of Native Americans from a planet ceded to the Cardassians.

Preemptive Strike S7E24 - Lieutenant Ro is sent undercover to root out a Maquis cell.

The Maquis Parts 1 & 2 S2E20 & 21 - When a Cardassian freighter explodes at Deep Space 9, the Cardassians blame Federation colonists in the new Demilitarized Zone. Sisko tries to rescue Gul Dukat, stop the Maquis terrorists, and prevent a new war with the Cardassians.

Colonialism

Up the Long Ladder S2E18: The Enterprise discovers two threatened colonies which must cooperate to survive. Duet S1E19: After a Cardassian man arrives on the station suffering from an illness that he could only have contracted at a Bajoran labor camp during the Occupation, Major Kira leads an investigation to determine whether he is actually a notorious war criminal. Cardassians S2E05: Garak and Doctor Bashir investigate abandoned Cardassian war orphans on Bajor. Alliances S2E14: After continuous Kazon attacks against Voyager cost the lives of several crewmembers and the damage to the ship continues to mount, Janeway seeks an alliance to secure Voyager's passage through the Kazon Collective.

 Aging

Firstborn TNG S07E21- A mysterious family friend and advisor encourages Worf's son Alexander to become a warrior.

Fury VOY S06E23- Kes returns, seeking revenge on both Voyager and Janeway.

Facets DS9 S02E25- Dax performs the Trill zhian'tara ritual, which allows her to meet her previous hosts.

The Deadly Years TOS S02E11- The Enterprise discovers a colony full of rapidly-aging scientists. Whatever caused the rapid aging afflicts the ship´s landing party as well. Kirk, Spock, McCoy and Scott are shocked to discover they are aging decades each day and will soon die unless a cure can be found. The unaffected Chekov may be their only hope for survival.

Dangers of Automation

The Ultimate Computer S2E24- The Enterprise tests a computer that, if successful, could replace Kirk as the captain.

Evolution S3E01- An obsessed scientist arrives on the Enterprise-D to perform a once-in-a-lifetime experiment. Accidentally released nanites, however, threaten both it and the ship.

Learning Curve S2E16- Tuvok conducts a training session for Maquis personnel, but the ship's bio-neural gel packs start to malfunction.

Bride of Chaotica! S5E12- Trans-dimensional photonic lifeforms on an exploratory mission become entangled in a war with the characters from Tom Paris' "Captain Proton" program after mistaking Voyager's holodeck for reality.