The movie is called "The Fall Of The Roman Empire" but it actually takes place in the 2nd century A.D., centuries before the actual fall of the Roman Empire. However, it describes events which will later lead to the actual collapse of the empire. The movie starts in the winter of 180 A.D.. As a reminder, 180 A.D. is the last year of Marcus Aurelius' reign (reign: 161-180 A.D.) with co-emperor Lucius Verus. Marcus Aurelius is in the northern territories on the Danube frontier with his troops fighting the German barbarians.
With him there is general Gaius Livius (played by Boyd). Livius is the lover of Marcus Aurelius' beautiful daughter Lucilla (Sophia Loren) and also a good friend of the emperor's son Commodus (Plummer). He is surprised when he hears that Aurelius wants to make him his heir. Emperor Aurelius has egalitarian ideals and dreams of the day when all men in Rome are granted equal rights. It is worth noting that Marcus Aurelius was historically known as the Philosopher and that he even wrote a book called Meditations.
Emperor Aurelius trusts that Livius can achieve this egalitarian ideal. When his brutal son Commodus finds out about his father's plan to disinherit him, he is very deeply hurt and his friendship with Livius becomes sour. Before he has a chance to announce his successor, emperor Marcus Aurelius is poisoned by Commodus' political friends. Commodus becomes emperor and Livius lets it happen as he knows he has no chance of ever taking power without Aurelius' backing. Note that Commodus was historically known as a brutal emperor who executed many Roman citizens, participated in gladiatorial combats and who had a decadent private life. All the previous emperors before him were known as the Five Good Emperors. In the movie, Commodus undoes all of Aurelius' policies (a historical fact as well). He taxes the provinces heavily in favor of Rome and Italy breaking with the previous tradition which consisted in treating the provinces as equals.
Livius is eventually recalled when a rebellion breaks out in Rome's eastern provinces. Livius then finds out with horror that Lucilla is behind the unrest. Lucilla tries to persuade Livius to form another state opposed to Rome but Livius who is loyal to Rome and who respects Roman civilization refuses. Livius crushes the rebellion and scores a victory.
Commodus sends word to Livius that he is to be made joint ruler of Rome. But he wants Livius to brutally punish the populations of the provinces. Livius refuses. He then returns to Rome with Lucilla and with his army and orders Commodus to abdicate. Commodus bribes Livius' soldiers and massacres the German populations thereby ensuring centuries of animosity between the Germans and Rome.
The Senate makes Commodus a god and sentences Livius and Lucilla to be burned alive as a sacrifice to the new god. Later Commodus challenges Livius to a duel. Livius kills Commodus and the Roman Senate offers Livius to become emperor which he refuses. In his eyes Rome has become too corrupt to rule.