Stardate 4040.7: The Enterprise is tracking down a missing ship, the Beagle, and find its wreckage but no crew. They follow the trail to a nearby planet where they pick up primitive "television" transmissions, talking about rebelling slaves and gladiatorial combat. A "barbarian" is killed in the games and Spock identifies the person as a crewmember on the missing ship.
Kirk, Spock, and McCoy beam down outside a large city, where they are ambushed by a group of runaway slaves led by the elderly Septimus and ex-gladiator Flavius, all of who worship the "Sun." Kirk convinces them they're from a "distant land" and they agree to send Flavius with them so they can look for the missing crew. Kirk mentions the captain of the missing ship, Captain R.M. Merik, who the slaves identify as Merikus, First Citizen. They still agree to help the Enterprise party but they are captured as they head for the city and the soldiers quickly recognize Spock as a "barbarian."
The trio is imprisoned with Flavius, who is led off to the games. They make a break for it but they are quickly recaptured and they meet… Captain Merik, now a First Citizen under the proconsul, Claudius Marcus. They have a private meeting and Marcus reveals that he knows who they are and what the Federation is, compliments of Merik. Merik reveals they beamed down to repair their ship and met Marcus, who convinced Merik that their planet shouldn't be "contaminated." Merik beamed down the rest of his crew, who were sentenced to fight in the arena. Marcus expects Kirk to beam down his crew as well and the captain refuses. Marcus threatens Spock and McCoy if Kirk doesn't call down a new landing party. Armed guards show up and Kirk gives Scotty a standby order. Kirk can't beam down an armed party or attack from orbit without breaking the Prime Directive, which Marcus well knows. Kirk reestablishes contact and gives Scotty a Condition Green message, and an angry Marcus has them taken to the arena.
In orbit, Scotty knows the Condition Green means they're in trouble but he's prohibited from taking action. Scotty orders Chekov to pinpoint the city's power sources.
Spock and McCoy are taken into the arena for the televised event while Kirk is forced to watch with Marcus and Merik. McCoy is forced to fight Flavius while Spock spars with another opponent. McCoy isn't doing well and Marcus notes that Spock can't help him. Kirk settles back and bluffs Marcus into believing he's unconcerned. When Flavius gets the upper hand on McCoy, Spock quickly drops his opponent and then takes down Flavius. The referee asks for Marcus to settle the matter and the proconsul decides to keep them alive for the threat of a more painful death later.
Spock and McCoy are taken back to their cell while Kirk is taken to Marcus' quarters where he finds a willing slave girl. She's there as his slave girl for the night. In the cell, Spock ceaselessly tries to get out of the cell and McCoy tries to apologize to him. Spock dismisses his attempt and McCoy points out that Spock is more willing to die rather then let his human half slip out. Finally McCoy admits he's concerned about Kirk.
Kirk spends the night with the slave girl, Drusilla, and later in the afternoon she leaves and Marcus greets him and notes one of their communicators are missing. Merik searches Kirk and doesn't find it, and Marcus dismisses him. He then notes he wanted to give Kirk one last night as a man before dying in the arena, with a quick clean kill. The Master of the Arena prepares to kill Kirk but Flavius bursts in. The guards gun him down and Kirk grabs a gun… and Scotty cuts the power generators from orbit. Kirk makes his way to the cells and frees Spock and McCoy but guards come at them from both side. Marcus orders them to go to swords to avoid a crossfire and he tells Merik to watch and see how men die. The Enterprise crew manage to hold them off but the guards close in… and Merik uses the stolen communicator to signal Scotty for beam up. Marcus stabs him in the back but Merik tosses Kirk the communicator and the landing party beam out as the guards open fire.
Kirk gives Scotty a commendation for his quick thinking and they consider the slaves' sun-worshipping belief. Uhura reveals that the "Sun" the slaves worship is the Son of God. The crew realize that just as on Earth, the Roman Empire will fall to a new more peaceful way.