Plot
In the winter of 180 AD, the ailing Roman emperor Marcus Aurelius fights to keep Germanic tribes from invading his northern territories on the Danube frontier. His deputies are the Greek ex-slave Timonides and the stern and honest general Gaius Livius. Privately Aurelius holds egalitarian ideals, and wants a successor who will reform the empire and grant equal rights to all its subjects; this disqualifies his son Commodus, who prefers to rule by force. Instead, Aurelius decides to nominate Livius, Commodus's closest friend and the lover of the emperor's daughter Lucilla.
Before Aurelius can announce his plan, he is poisoned by Commodus's crony Cleander, who with his allies hope to secure their own political future by ensuring Commodus' ascension. Feeling that a plebeian such as himself would never be accepted as emperor without Aurelius's explicit backing, Livius lets his old friend take the position instead. Commodus is not part of the murder plot, but nonetheless hates his father for trying to deny him the throne. He dedicates himself to undoing all of Aurelius's policies of equality, and to extracting heavy taxes from the provinces to enrich the city of Rome.
Meanwhile, Livius's army defeats the Germans on the frontier. Among the captives are the chieftain Ballomar and his court. Timonides and Lucilla convince Livius to advocate for mercy towards the conquered barbarians, thus perpetuating the legacy of Aurelius. Timonides wins the Germans' trust by successfully undergoing an ordeal, having his hand held in the flame of a torch without crying out, and they agree to submit to the judgement of the Roman Senate. Despite hostility from Commodus, speeches by Livius and Timonides persuade the senators to let the German captives become peaceful farmers on Italian land, thereby encouraging their fellow barbarians to cooperate with Rome instead of fighting it. Thwarted, Commodus sends Livius back to the northern frontier and Lucilla to Armenia, with whose king, Sohaemus, she shares a loveless political marriage.
Lucilla joins a revolt in Rome's eastern provinces, where a famine has been exacerbated by the new taxes. Commodus sends his northern army against the rebels, knowing that Livius will put aside personal feelings and fight to preserve the unity of the Empire. As the opposing Roman armies meet for battle, Sohaemus arrives and attacks Livius with both the Armenian army and troops borrowed from Rome's archenemy, the Persians. The rebels patriotically decide to fight Persia instead of Rome, joining with Livius and helping him to vanquish Sohaemus. As a reward Commodus declares Livius his co-emperor, but only on condition that the northern army inflicts harsh punishments on the rebellious provinces.
Rejecting this piece of brutality, Livius and Lucilla take their army to Rome, intending to make Commodus abdicate. The emperor responds by bribing away the soldiers' loyalty and massacring Timonides and the population of the German colony. Lucilla tries to hire Verulus, Commodus's gladiator bodyguard, to assassinate her brother; Verulus declines, confessing that he slept with Aurelius's wife and that Commodus is his illegitimate son.
The Senate declares Commodus a god, and Livius and Lucilla are sentenced to be burned alive as human sacrifices to the new deity as the Roman citizens drunkenly celebrate. In consideration of their former friendship, Commodus offers Livius a duel for the throne. The two fight with javelins in the Roman Forum, where Livius eventually runs Commodus through. At last Livius is free to become emperor, but he has lost faith in Rome's ability to reform. He departs the city with Lucilla, as Commodus's old advisers fight over the throne and offer competing bribes to the army in an attempt to gain military support.
A voice-over epilogue states that though the Roman Empire did not fall immediately, internal corruption led to its eventual collapse.