The U.S.S. Enterprise finds the wreckage of the S.S. Beagle, with no survivors, orbiting near the Planet 892-IV. When Kirk, Spock, and McCoy beam to the planet's surface, they find a disheveled group of "sun worshipers" trying to escape being caught by the local authorities. They are captured, along with the landing party, and taken to the city which looks very much like Earth's ancient Rome.
They find that Captain R.M. Merrick of the Beagle has betrayed his crew, beaming them down to fight in the Roman-like gladiatorial games. Merik is First Citizen of the Empire and supposed close friend to the Proconsul, Claudius Marcus. Kirk realizes, however, that Merrick is being used as a lure to get more starship crews to 892-IV for the entertainment of its inhabitants.
When Kirk refuses to beam his crew down to die in the arena, Spock and McCoy are condemned to fight gladiators in the ring. The bout is being televised for the planet's enjoyment, but Scotty cuts off the planet's energy supply, spoiling their pleasure. When Kirk uses the confusion to free Spock and McCoy, Merrick sees how a true starship captain acts in the face of danger and uses his communicator to have Kirk, Spock and McCoy beamed aboard the U.S.S. Enterprise. For his treachery, Merrick is killed by the Proconsul.
Uhura, who has been monitoring the radio waves of the planet, realizes that the "sun worshipers" aren't talking about the sun in the sky, but of the "Son of God."