It is the beginning of the third century and Rome is in crisis. Ravaged by civil war and foreign invasions, it now faces an even greater challenge - the new religion of Christianity. When barbarian Goths attack the Empire's borders, and traitors rise against Emperor Philip, his trusted general Decius blames Philip's leniency toward the Christians. But Decius soon becomes a traitor, as well, as he faces Philip in battle, taking the crown for himself. As emperor, Decius sets out to win back the pagan gods' favour, and his reign's first victims are the Christians. Yet the wrath of the gods only continues as plague strikes and the Goths launch full-scale invasions into the Empire. Decius and his son, Herennius, are forced to meet the Gothic king, Cniva, in a deadly clash that will mark the first time a Roman emperor is ever killed by a barbarian in battle. Unfortunately for the Empire, it will not be the last.

Near the end of the third century, civil war has torn the empire apart and left its borders vulnerable to barbarian invasion. Romans living on the eastern and western frontiers, far from Rome, are left to defend themselves, which they do by building their own armies and naming their own emperors. As the Empire divides against itself, one extraordinary man rises from the rank of soldier to become emperor in Rome. His name is Aurelian, and with an unshakeable trust in the sun god, he fights back the barbarians who by now threaten Rome itself. Once he secures the great city, Aurelian pursues the rulers of the splinter empires: an eastern warrior queen and a proud western general. Defeating both, he reunites the empire in five short years, earning the title Restorer of the World. For a moment there is peace. But in 275 AD Aurelian is assassinated; his death is mourned by Romans across the empire, who know too well the scarcity of greatness in their rulers.

Despite the Empire's successes, the extensive military campaigns begin to take their toll on Rome's finances. When Dacian tribes ravage Roman lands, the unpopular Emperor Domitian is forced to raise taxes. He has never been on the front lines himself, but one of his generals, a young man named Trajan, soon proves to be a worthy leader while battling Germanic tribes on the Rhine. After Domitian's murder, Trajan is proclaimed emperor and sets out to rebuild the troubled Empire. He solves its financial problems by attacking the Dacian chieftain, Deceblas, uncovering his golden treasures, hidden in a river. In 106 AD, Deceblas flees, but Trajan's men hunt him down, only to find that he has killed himself to avoid the humiliation of defeat. Trajan's victory is immortalized in Trajan's Column in Rome, and under his reign, the Empire reaches its maximum extent of power.

It is 162 AD. Marcus Aurelius inherits the empire at its peak. Peace and prosperity have reigned for a hundred years, and the imperial boundaries stretch from the Irish Sea to the sands of Syria. But the greater Rome grows, the bigger a target she becomes. War breaks out on two fronts - in the east and in the north - just as plague takes a terrible toll on the Empire's population. Marcus Aurelius would rather stay home and write about philosophy, but duty dictates that he save the empire. His German wars are a prolonged, bloody quagmire that seem to stretch on without end. But with his death, his long dream of conquering the German lands is lost, when his son, Commodus, fails to pursue the mission, preferring the luxurious life in Rome to the dangerous life of a soldier.

It is 60 BC. Over-powerful generals and money corrupts the Roman Republic. The empire churns with civil war, and violence and murder run rampant in the streets. Julius Caesar, desperate for fame and honor, embarks on a brutal decade long campaign to annex Gaul and build his own reputation. His aristocratic rivals try to stop him, but he is an expert manipulator. His propaganda and extravagant victories against bloodthirsty barbarians thrill the public. In 49 BC, he crosses the Rubicon intending to claim his rightful place as the chief man of Rome. It sparks a new civil war that pits him against Pompey the Great and sounds the final death knell for the Republic.

It is 47 AD. The new emperor Claudius has lost the respect of those he seeks to rule. He can only hang onto his throne by waging a spectacular conquest. He chooses Britannia, a legendary land which fascinates and inspires fear in the Romans. Proud and warlike, the Britons use the island's treacherous terrain and dark forests to lure the legions to ruin. Led by magic wielding Druid priests and a charismatic warrior named Caratacus, the Britons resist in the face of the greatest military machine the world has ever known.

As emperor of Spain, Gaul, and Britain, Constantine proves his strength against his barbarian enemies. But when the other Roman emperors begin to turn on each other again, Constantine determines to put an end to such rivalries once and for all. Defeating emperor after emperor, Constantine slowly gains control of the entire Roman Empire and establishes a new capital in the east. Along the way, he has a vision that will change the course of western civilization forever. Fighting under the cross of the Christian God, Constantine not only legitimizes the young religion, but seeks to use it as a unifying force in the empire.

By the end of the fourth century, Romans and barbarians live together uneasily in the empire, a situation that often explodes into violence. When Emperor Theodosius enlists the Goths as mercenaries, he relies on his trusted general, the half Vandal, half Roman Stilicho, to ensure the Goths loyalty. But Theodosius uses the Goth soldiers as canon fodder in a civil war, causing them to rebel under the leadership of Alaric, a man they call king. Theodosius dies soon thereafter and leaves the Empire to his two young sons, naming Stilicho as regent in the west. As Alaric seeks to find a home for his people in the Empire and Stilicho fights to protect that empire from new enemies, the two men cross paths, sometimes as allies, sometimes as enemies. But in the end, both will fall victim to Roman prejudice. And the Goths, realizing that Romans want nothing to do with barbarians, will strike back against Rome itself.

In 450 AD, Rome is a mere shadow of its former self. Gone is the superior civilisation and great expanses of land. Now, Roman rulers are at the mercy of their barbarian invaders, offering them power and territory in exchange for peace. In the midst of the Empire’s low decline, three comrades rise in the ranks of the Roman military: Ricimer, Aegidius and Majorian, all competing for Imperial control. Majorian is the first to be named emperor, and Aegidius is made his prime general, but Ricimer, a soldier of barbarian descent, is the real puppet master. Eventually, any man who threatens his own authority, including his friends, will meet their bloody end at the edge of his sword.

In the middle of the fifth century, as the Empire fights a losing battle against its formidable barbarian opponents, one Roman named Orestes finds service in the court of Attila the Hun. Schooled in the strategy of his enemy, Orestes eventually makes his way back to Italy, determined to resurrect Rome. Naming his young son emperor, Orestes attempts to purify Rome of her barbarian influences. Yet he soon learns the sad truth, that without barbarians, there is no Rome. After hundreds of years of using the foreign invaders as mercenary soldiers, they outnumber the Romans, and the Empire can’t afford to dismiss them. Though Orestes fights valiantly, he meets his end at the hands of the leader of the barbarian mercenaries, Odacer, an event that spells the end of Rome in 476 AD. The last emperor, just a boy, is sent into exile, and Odacer proclaims there is no need for a new emperor, as the Roman Empire is no more.

It is 113 BC Rome is a Republic, a small empire that clings to the rim of the Mediterranean. Though a democracy in name and spirit, a man must be rich and from the noble class to hold the highest offices, both political and military. The system has worked well for three centuries. But now a barbarian horde known as the Cimbri smashes through the northern imperial border. One humiliating defeat follows another, with losses of hundreds of thousands of Roman legionaries. Terror grips Rome and drives her into the arms of General Marius. Though he is a commoner, he has a brilliant military mind. To defeat the Cimbri, Marius will transform the Roman army and shake the Republic's political foundations to the core. It is a turning point for the Republic. Faced with the savage Cimbri, Rome must decide between dictatorship or annihilation.

When Roman generals Pompey and Crassus lead their mighty legions of soldiers and mercenaries into the lands surrounding Italy, neither could anticipate the turmoil caused by one bold mercenary: Spartacus. A Thracian warrior, Spartacus deserts the Roman army, but is captured and made a slave, forced to fight as a gladiator. But in 73 BC the untamable barbarian leads a slave revolt of 70,000 gladiators against the Roman Republic. Though the brutal and conniving Roman General Crassus is finally able to suppress the revolt, his rival, the more popular Pompey, takes all the credit, sparking a division within the Republic itself that will ultimately spell its demise.

As the Roman Empire expands its reaches into what is now Germany, its legions are faced with ferocious Germanic tribesmen. In 9 AD, their chieftain, Arminius, raised in Rome as a hostage of peace, uses his Roman military training to slaughter the Roman General Varus' army at Teutoburg Forest. This landmark battle establishes the Rhine River as the frontier between the Empire and its growing barbaric enemies. Six years later, when General Germanicus sets out across the Rhine for revenge, he finds heaps of bones and severed skulls nailed to trees - a reminder of the barbarians' victory. But at the Battle of the Weser River, he faces Arminius and wins. Germanicus captures Arminius' pregnant wife, Thusnelda, and parades her through the streets of Rome, as a symbol of his triumph.