Following the recent receipt of a message sent by U.S.S. Horizon, which was lost with all hands over a hundred years previously, the Enterprise is visiting the planet Sigma Iotia II. On arrival in orbit, Uhura receives a message from someone called Bela Oxmyx. Kirk explains why they are there and Oxmyx promises a welcoming party when they beam down. Kirk reminds Spock and McCoy that the Horizon came into contact with the planet's inhabitants before the instigation of the Prime Directive and that there is a chance that their culture may already have been contaminated.

They arrive in a busy street that resembles 1920s Earth and everybody around them looks to be carrying a gun of some sort. Two men carrying machine guns order Kirk and Spock to put their hands on their heads before removing their communicators and phasers. As they are being taken to Oxmyx at gunpoint, the occupants of a passing car open fire on them, killing one of the men. The other, called Kalo, continues on as if nothing out of the ordinary has occurred. Oxmyx meets them in a large room, hears of the incident, and orders Kalo to make a retaliatory 'hit' on rival gang leader Jojo Krako. On looking around, Spock spots a large white book entitled 'Chicago Mobs of The Twenties' which was probably left behind by the crew of the Horizon and seems to be the source of contamination around which the local inhabitants have built an entire culture.

Oxmyx tries to make a deal with Kirk to obtain weapons but Kirk refuses, at which Oxmyx threatens to kill them all if he doesn't change his mind within eight hours. Kalo brings out their confiscated hardware and Kirk is forced to explain their function to stop them blowing themselves up. While the landing party are being led away and locked up Oxmyx uses a communicator to explain the situation to a very confused Scotty aboard the Enterprise. Kirk muscles in on a game of poker being played by their guards and introduces them to a very confusing game called 'Fizzbin', the rules of which he makes up as he goes along. While the guards are being distracted by Kirk's explanations Spock and McCoy manage to overpower them.

Kirk despatches his companions to the local radio station to try and get a message through to the Enterprise while he goes after Oxmyx. Spock and McCoy succeed in managing to get themselves beamed back up to the ship but Kirk is captured by two of Jojo Krako's men. Krako makes much the same offer as Oxmyx but adds that he will cut Kirk in for a third of 'the action'. Kirk suggests trying a peaceful solution instead, which annoys Krako who has him locked up. On board the Enterprise Spock and McCoy have been unable to discover a precedent for handling their current situation so, when Oxmyx contacts them and offers to help rescue Kirk, Spock agrees.

The main phaser banks on the ship are set to 'stun' after which McCoy and Spock beam back down, only to be recaptured by Oxmyx and his men. In the meantime, Kirk has managed to effect an escape by himself and, walking into the room where the two are being held, he turns the tables on their guards, taking their clothes and weapons and heading back to Krako's. With the help of a young boy they get into Krako's H.Q. and Kirk tells him that the 'Federation' is taking over. If Krako helps, he might be cut in for a percentage. He explains that they want one man to rule the planet with the Federation 'pulling his strings'. Krako agrees, on which Kirk sends a coded message to the Enterprise and Krako is beamed aboard.

They return to Bela Oxmyx where Kirk makes the same demands. Oxmyx also agrees and starts to call the other gang bosses to arrange a meeting. As each one answers, Scotty locks onto them and beams them straight into Oxmyx's office. Kirk tells them that they are all going to combine in a single operation with the Federation taking a forty percent cut. Krako's men attack the building but are knocked out by the Enterprise's phaser banks. With the bosses at his mercy, Kirk arranges for Oxmyx to be the top boss with Krako as his first lieutenant. He says that the Federation will stop by every year to collect their 'piece of the action'.

Back on board the Enterprise Spock is keen to know how Kirk will explain to Starfleet why a ship will need to be sent every year to collect their 'cut'. McCoy sheepishly admits that he thinks he left a communicator in Oxmyx's office. Kirk is sure that the Iotians will figure out how it works and half-jokes that in a few years 'they may be demanding a piece of OUR action'.