The series begins in 51 BC, with Gaius Julius Caesar's masterful conquest of Gaul. After eight years of war, and loaded with loot and slaves, he prepares to return to Rome, triumphantly accompanied by thousands of loyal battle-hardened men, and a populist agenda for radical social change. Terrified, the aristocracy threatens to prosecute Caesar for war crimes as soon as he sets foot in Rome. Caesar's old friend and mentor, Pompey Magnus, attempts to foment civil war in order to maintain the balance of power.

Two of Caesar's soldiers, Lucius Vorenus and Titus Pullo, thwart Pompey's plan and in the process, win the eternal gratitude of Caesar and the Julian clan. From this gratitude comes access, and the two plebian officers are granted an intimate view of the ruling class. The fates of Pullo and Vorenus become entwined with those of Caesar's great nephew, Octavian. An awkward but precocious child who, with prodding from his ever-scheming mother, he will become the first emperor of Rome.