The Enterprise receives a 100 year old radio transmission from the U.S.S Horizon on the planet Iotia and investigates. The person they first contact is "Boss" Oxmyx. Kirk, Bones and Spock beam down to Iotia to find a world very much like 1920s Chicago, only with people not even trying to hide the fact that they're carrying sub machine guns. Apparently, shoving one in somebody's face is their way of saying "Hi".It seems that when the Horizon visited Iotia 100 years ago, they left behind some books. Aside from being virtually identical to humans, Iotians are very imitative and adaptable by nature. There were some textbooks on how to build radios and such, plus a nice little volume called Chicago Mobs of the Twenties, copyright 1992. The Iotians have made this book on '20s mobsters the basis for their culture. And Boss Oxnyx wants "heaters" (phasers) or it's curtains for Kirk.
- Are You Sure You Can Drive This Thing?: Kirk drives one of the 1920s-model automobiles based on his and Spock's vague recollections of how such vehicles work.Spock: Captain, you are an excellent starship commander but as a taxi driver you leave much to be desired.
Kirk: It was that bad?
- Bald of Evil: Krako has a noticeable bald spot, and he's one of the most ruthless bosses.
- Bizarre Alien Psychology: The Iotians likely come as close to this trope as TOS ever got. They take mimicry of other cultures to extremes far beyond anything humans have done, even with the inspiration being so obviously detrimental to themselves to the point that everyone in their society wants to get shot at for no appreciable gain.
- Brick Joke: Becomes one in DC's Star Trek: Remember McCoy's communicator? Bella returns it intact, figuring McCoy wouldn't like him messing with it.
- Calvin Ball: The card game "fizzbin".
- Chain of Deals: Star Trek 25th Anniversary for the NES has a segment based on this episode and was structured this way. In the actual episode, Kirk and co. refuse a deal to give them phasers. They're trying to undo a mistake that was made by giving them advanced tech, after all!
- Chekhov's Gun: With an actual gun—Spock tells Scotty to keep the ship's phasers on stun (perhaps the only time they do that). Kirk then has Scotty fire them to stop a shootout and show everyone what they're dealing with.
- Corpsing: At one point Spock gets tangled in a bit of mobster slang and you can see Leonard Nimoy - well, not cracking up, because this is Spock, but definitely struggling not to smile.
- Dart Board Of Hate: Krako has one of Oxmyx. Of course, he cheats at darts.
- Death from Above: This is one of the few shown cases of orbital bombardment in Star Trek, though it is not death from above, just stunning (yes, even the Federation's starships have a stun setting on the phasers).
- The Don: Several Dons. In the end Kirk is the biggest Don on the planet — and he makes clear that he's really only an enforcer for the real mob, the Federation.
- Dramatic Gun Cock: As Kirk and Spock confront Krako, Krako's men get the drop on them this way.Spock: Captain, I believe that perhaps it would be wise to do as he says. I just heard the sound of...
Kirk: The sound of a machine gun bolt being pulled back.
- Drives Like Crazy: Kirk is an awesome Starship Captain, but a lousy driver.
- Forgotten Phlebotinum: Who knew that the ship's phasers could be set on stun? This incredibly useful feature had never appeared before and would never be used again across the entire franchise!
- Gangland Drive-By: The crew land on a planet who based their society on 1920's gangsters, complete with period weapons and vehicles. Minutes after beaming down, a drive-by takes place right in front of them.
- Gangster Land: This planet's hat.
- Giving Radio to the Romans: That's what started this mess!
- Giving Up on Logic: Even Mr. Spock (gasp!) gives up logic due to the fact that he's observed that normal rules do not apply on this planet.
- "I would advise youse to keep dialin', Oxmyx."
- Gunboat Diplomacy: "The Federation's takin' over whether you like it or not!"
- Had the Silly Thing in Reverse: Kirk, every time he tries to drive the car.
- Here We Go Again: Bones accidentally leaves his communicator behind.
- Hidden Depths: Despite being a mob boss, Oxmyx admits that he doesn't like everyone trying to knock off everyone else.
- I Lied: When Oxmyx tricks Spock and Bones into beaming back down and recaptures them:Spock: Mr. Oxmyx, I understood that we had an arrangement, a truce.
Oxmyx: I was hoping you'd think that, dummy.
- Large Ham: Kirk has way too much fun playing gangster. And then there's Krako, who seems to have No Indoor Voice.
- Locking MacGyver in the Store Cupboard: Kirk manages to escape using some wires from a radio, a bed sheet and a waste basket.
- Mundanization: Kirk and co. are beamed down in the middle of a world of crosswalks and fireplugs.
- Nice Job Breaking It, Hero!: Whoever left that book about the gangsters behind.
- Offered the Crown: At the end, Oxmyx wants Kirk to be the top boss. Kirk declines and makes Oxmyx the top boss, with Krako as his Number Two.
- Oh, No... Not Again!: Krako's Number Two, when he wakes up after Kirk knocked him out for the second time.
- O.O.C. Is Serious Business: See Giving Up on Logic above.
- Planet of Hats: Check out the page for this trope. This episode is the poster child! Incidentally, if you look closely, you'll notice the three gangs pictured wear three different kinds of hats. Oxmyx's gang all wear fedoras. Krako's gang all wear boaters. Mirt's gang all wear bowlers.
- The Syndicate: Part of Kirk's solution for the mess is forcing the Iotian mobs to form one (the idea being that they'll learn to work together instead of constantly fighting each other), kept together by the threat of the Federation intervening.
- Time Line Altering Mac Guffin: Chicago Mobs of the Twenties, copyright 1992. (Probably sounded more impressive in January of 1968)
- We Need a Distraction: Will this Street Urchin do? He can even give us a Title Drop!
- What Happened to the Mouse?: The Street Urchin mentioned above never returns to claim the "piece of the action" that Kirk and Spock promised him.
- His "piece" was the beautiful dramatic act he gave that let Kirk and Spock into the building. As they enter, Kirk says "A piece of the action!" and the kid smiles and runs off.