Having been lost six years previously, the fate of the S.S. Beagle is discovered by the Enterprise when it comes across wreckage drifting in space. Kirk was at the Starfleet Academy with its captain, R.M. Merrick, and as no bodies are found, decides to investigate further. The drift of the wreckage is traced to a point of origin near planet IV in system 892. On approaching the planet, Uhura picks up a television transmission showing a gladiatorial fight in which a William B. Harrison 'the last of the barbarians' is killed.

Spock identifies the name as being that of one of the crewmembers of the Beagle. Kirk, McCoy and Sock beam down to a location outside the main city, which seems to be similar to that of 20th century Earth, and are almost immediately fired upon. A group of renegade slaves, led by a man called Flavius, takes them to see Septimus, their leader, from whom they find out that the culture is almost identical to that of ancient Rome which has never fallen on this planet. Septimus and his people are all escaped slaves who worship the sun. They have never seen men like Kirk and are shocked when he mentions Merrick's name.

The First Citizen is called Merrickus, a cruel butcher who is their sworn enemy. Kirk, thinking quickly, tells them that he has come to take Merrickus away for punishment and Septimus agrees to his request for help. After changing into local clothing they are taken to the city but are caught by a guard patrol outside the walls and taken to prison cells inside. Kirk tells one of them to get a message to Merrickus that Jim Kirk wants to speak with him. When the guard returns, Flavius is told that he has been chosen to compete in the morning games. He insists that he will refuse to fight but is taken away into the city. The Enterprise men manage to overcome their guards but, on reaching the corridor, are confronted by Merrickus, Proconsul Claudius Marcus and six armed guards.

They are taken to the Proconsul's rooms and are surprised to discover that he knows exactly who and what they are. Merrick explains that the Beagle came to the planet seeking materials to repair the ship after a meteor strike. Claudius Marcus persuaded him to stay in order to prevent other cultures from contaminating their civilisation, following which Merrick ordered his crew to beam down and killed any of them who wouldn't co-operate. The same threat is made to Kirk and he is handed a communicator to contact the Enterprise. Before being taken away he tells Scotty that they are 'condition green' which is a code term meaning that they are in trouble but not requiring any immediate action from the Enterprise.

Despite this, Scotty starts preparing a few surprises for the planet's inhabitants anyway. The gladiatorial fights are major prime time TV and at the top of the bill are Spock and McCoy who have been matched against Achilles and Flavius. The fight is half-hearted until the other prisoners are threatened at gunpoint. Spock quickly overcomes the unfortunate Achilles and sees off Flavius with a Vulcan neck pinch. Spock and McCoy are taken back to their cells and Claudius takes Kirk to his chambers where he presents him with a beautiful slave, Drusilla, to do his bidding. Later, Kirk is awoken and searched for a missing communicator which fails to turn up.

He is taken to the arena to fight Flavius, who attacks the Games executioner and is shot for his trouble. At that moment, Scotty uses the Enterprise to disrupt power on the planet, giving Kirk the chance to escape and free Spock and McCoy. They attempt to get away but are trapped by guards and forced to fight with swords. Merrick uses the stolen communicator to contact the Enterprise and is stabbed by Claudius. Dying, he throws the device to Kirk, allowing Scotty to lock onto them and beam them up to safety. Later, on the Enterprise, Uhura suggests that the slaves were misunderstood when they said they worshipped the sun. What they had actually meant was the planet's equivalent of a Christian belief, 'the Son' - of God.