With the TARDIS stuck at the bottom of a cliff, the four time travellers have installed themselves in an unoccupied Roman villa. The owner, Flavius Giscard is away campaigning in Gaul. As the Doctor and Ian recline, Barbara and Vicki walk to the nearby Roman village. At the market they are spotted by two slave traders, Didius and Sevcheria. When they return to the villa the Doctor announces that he is off to Rome, some miles away, and will travel there with Vicki. Later that evening Barbara and Ian, now alone, are relaxing when the two slavers burst in upon them. They are soon overpowered and taken prisoner. Ian is sold to one slave owner, while Barbara is to be traded with another and sent to Rome.
The Doctor and Vicki are en route for Rome when they find the murdered body of a lyre player named Maximus Pettulian. The Doctor is holding the man’s lyre when a Centurion arrives and mistakes him for the dead man who is late for an engagement in Rome. The Centurion thus accompanies them to Assysium. Once stationed at an inn there, the Centurion makes contact with the mute assassin Ascaris, who killed the real Pettulian, and instructs him to kill the Doctor. The assassin draws his sword and heads off to the Doctor’s chambers.
The Doctor overpowers the assassin and, along with Vicki, drives him away through an open window. It seems the Centurion has fled, and the Doctor concludes the soldier was in league with the assassin. He decides to maintain his alias as Pettulian and head onward to the city of Rome. Barbara is meanwhile already in the city and is soon sold in open auction for 10,000 sesterces to a man named Tavius, who is highly placed in the court of the Emperor Nero. She is to be a handmaiden to Nero's second wife, the Empress Poppaea Sabina. Tavius is a kindly man but warns that if she tries to escape her slavery that she will be killed.
The Doctor and Vicki arrive at Nero’s court too and encounter Tavius, who seems to imply to the Doctor that Pettulian is part of a secret network in which he is also a player. Further discussion is interrupted by the arrival of Nero himself, a laughable excuse for a leader who seems arrogant, vain and selfish. The Doctor cleverly avoids an extended lyre concert, and then has the freedom of the court. On one walk around they find the body of the Centurion who imperilled them earlier.
Ian has been confined to a galley in the Mediterranean but the craft soon runs into rough seas and is broken up. He is washed up on the nearby shore and there is found by another survivor of the galley, Delos, who has saved his life and removes the last of his chains. They agree to head for Rome in search of Barbara. When they reach there, however, they are captured by some centurions. Taken to the arena they are set to be trained as gladiators – and their first opponents will be the lions.
It becomes apparent to the Doctor that Tavius had the Centurion murdered and that he too is expected to undertake some sort of action. Nero decides the Doctor must fulfil an obligation too, and organizes a banquet in his honour at which he must play the lyre. He also takes a shine to Barbara and starts to pursue her romantically – and literally – much to the anger of Poppaea, who decides to have her poisoned at the Pettulian banquet. Barbara has just left the banquet chamber when the Doctor arrives there, warning Nero that he has learnt his wine could be poisoned. It has been, as part of Poppaea’s plan.
The Doctor is soon put to perform centre stage and picks up his lyre with the warning that only those with the most sensitive and perceptive hearing will be able to discern its subtle melody. He then creates absolutely no sound but has created a climate in which no-one wishes to make themselves out to be philistines by not appreciating the music. Nero is not convinced, however, and in private fumes against the deception. He decides to have Pettulian fed to the lions.
Meanwhile, at the arena itself Ian and Delos have been trained as gladiators and are set to fight each other. With Nero watching them they are told to battle to the death.
However, Delos and Ian decide to fight their way out of the arena, and Ian is able to shout to the watching Barbara that he will be back to rescue her before he and Delos flee. The Emperor calls off his soldiers when it becomes clear they cannot be caught, planning to have him killed when he returns to rescue Barbara. A crowd of soldiers are arranged at the palace.
The Doctor has meanwhile found the architectural plans for Nero’s new Rome, and deduces that since the year is 64 AD that the Emperor is planning to destroy the city. Tavius arrives and warns the Doctor that the Emperor is planning to kill him too, advising him to complete his mission and kill Nero soon. It seems that Pettulian was an assassin all along. The Doctor and Vicki decide to leave quickly but before departing accidentally set fire to Nero’s architectural plans. The Emperor notices this and decides to burn down the city, thanking the Doctor and deciding after all to spare his life. A rabble are bribed into starting the blaze and while anarchy rages Ian is helped into the palace by Tavius, who reunites him with Barbara. Under Tavius’ eye the two are allowed to escape and make their way from Rome and back to the villa; Tavius is revealed in the end to be an early Christian. Delos helps them get clear of the palace, parting from his friend Ian. The Doctor and Vicki also escape the city, watching it burn from a nearby hill.
By the time the Doctor and Vicki return to the villa, Ian and Barbara have spruced themselves up, and the Doctor mistakenly assumes that they have not even left the villa. All four leave in the TARDIS but have barely begun to travel when a strange force starts dragging the ship to an unknown location.