A planet where Rome never fell treats Kirk and his officers as gladiators and prisoners.
Searching for the lost survey ship Beagle, the Enterprise finds planet 892-IV, where the Roman Empire never fell and has reached a twentieth-century level, complete with gladiator games on television. Kirk, Spock and McCoy are captured, and they learn that the Beagle's captain, R. M. Merik, once known to Kirk at the Academy, now serves the regime as First Citizen.
Spock and McCoy are forced to fight in a televised arena while Kirk is held by the proconsul Claudius Marcus. Merik eventually helps them escape and is killed. Afterward Uhura realises that the slaves' peaceful faith is devoted not to the sun but to the Son of God.