In the winter of 180 AD, the Roman emperor Marcus Aurelius fights to keep Germanic tribes from invading his northern territories on the Danube frontier (a war which in fact had been ongoing for over a decade, with no end in sight as of 180). His deputies are the Greek ex-slave Timonides, a closet Christian, and the stern and honest general Gaius Livius. Livius has close connections with the imperial family, being the lover of Aurelius' philosopher daughter Lucilla and a friend of her brother Commodus. Nevertheless, he is amazed to hear that Aurelius wants to make him his heir. Despite his military obligations the emperor has egalitarian ideals, dreaming of a day when Rome grants equal rights to men of all nations. He knows that he will not live to achieve this end, and trusts Livius to do so more than his charismatic but brutal son. The discovery that his father has effectively disinherited him hurts Commodus immensely, and damages the almost brotherly relationship he had enjoyed with Livius.
Aurelius summons all the governors of the Roman Empire to his headquarters, intending to announce Livius' future accession. Before he can do so he is poisoned by Commodus' cronies, who hope to secure their own political future by putting their friend on the throne. Sure enough, Livius feels that a non-aristocrat such as himself would never be accepted as emperor without Aurelius' explicit backing; he lets his old friend take the position instead. Commodus, who was not part of the murder plot, is left feeling helplessly angry at his deceased father. He dedicates himself to undoing all of Aurelius' policies; this involves blatant favoritism towards Rome and Italy, which are enriched by ferocious taxation of the provinces that were to be their equals.
Meanwhile, Livius' army scores an important victory on the frontier, capturing the German chieftain Ballomar and his aides. Timonides wins the Germans' trust by successfully undergoing an ordeal, having his hand thrust in a fire; with his help, Livius decides to put Aurelius' policy into effect despite disapproval from Commodus. Lucilla helps convince Livius to defy the emperor, since she loved her father as much as Commodus hates him. A speech by Timonides persuades the Roman Senate to let the German captives become peaceful farmers on Italian land, thereby encouraging their fellow barbarians to cooperate with Rome instead of fighting it. Outraged, Commodus sends Livius back to the northern frontier and Lucilla to Armenia, with whose king, Sohaemus, she shares a loveless political marriage.
Lucilla organizes 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 Roman Empire. As the opposing Roman armies meet for battle, Sohaemus arrives, uninvited, and attacks Livius with both the Armenian army and cavalry 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 hideous punishments on the provinces that rebelled.
Rejecting this latest piece of brutality, Livius and Lucilla take their army to Rome and order Commodus to abdicate. He responds by bribing away the soldiers' loyalty and massacring Timonides and the population of the German colony (the latter action ensuring centuries of future hostility between Romans and Germans). The fawning Senate declares Commodus a god, and Livius and Lucilla are sentenced to be burned alive as human sacrifices to the new deity. This victory for Commodus is accompanied by a terrible private discovery – he is not of royal blood, being the product of illicit sex between his promiscuous mother Faustina Minor and the gladiator Verulus, who has since served as the emperor's bodyguard. His mind unhinged by this great shame, Commodus makes the bizarre decision of challenging Livius to a duel for the throne. The two fight with javelins in the Roman Forum, and Livius eventually runs Commodus through. The Senate hastily offer to make Livius emperor, but he refuses; the Roman government is now too corrupt for him to fix. He slips away with Lucilla, leaving Commodus' old advisers to bicker about who will take the emperor's place.
A voice-over epilogue states that this political infighting continued for the rest of Roman history, leading to the imperial government's eventual collapse.
Sophia Loren as Lucilla
Stephen Boyd as Livius
Alec Guinness as Marcus Aurelius
James Mason as Timonides
Christopher Plummer as Commodus
Mel Ferrer as Cleander
Omar Sharif as Sohaemus, King of Armenia
Anthony Quayle as Verulus
John Ireland as Ballomar
Eric Porter as Julianus
Finlay Currie as Senator
Andrew Keir as Polybius
Douglas Wilmer as Pescennius Niger
George Murcell as Victorinus
Norman Wooland as Virgilianus