The rebellious Thracian Spartacus, born and raised a slave, is sold to Gladiator trainer Batiatus. After weeks of being trained to kill for the arena, Spartacus turns on his owners and leads the other slaves in rebellion. As the rebels move from town to town, their numbers swell as escaped slaves join their ranks. Under the leadership of Spartacus, they make their way to southern Italy, where they will cross the sea and return to their homes. Meanwhile, in Rome, the slave revolt has become a deciding factor in the power struggle between two senators: the republican Gracchus and the militarist Crassus, each of whom sees the fortunes of the rebellion as the key to his own rise to power or humiliating defeat. As the two statesmen attempt to aid, hinder and manipulate the rebels for their own benefit, Spartacus and his followers press on toward freedom.

During the last century of the Roman Empire's rule, thousands are born enslaved to either the privileged class known as patricians or the wealthiest of the commoners, known as plebeians. One exceptionally strong slave in the rock mines of Libya, Spartacus, is regularly whipped for displaying his intelligence and pride. One day, Batiatus, who trains slaves to become gladiators, purchases Spartacus and several other slaves for his training camp in Capua. There, Batiatus announces that each man will be taught to fight to the death strategically, for the pleasure of patricians who enjoy the "sport." Training proves as dehumanizing as the mines; each slave is branded, mercilessly instructed by head trainer Marcellus, and kept in cells. Spartacus tries to befriend Ethopian gladiator Draba, but soon learns that the men refuse to ally, knowing that they may be forced to kill each other. One night, Spartacus is presented the slave woman Varinia. Batiatus and Marcellus, knowing that Spartacus has never had a woman, watch from a grate above his cell as Varinia stoically undresses. Their laughter disgusts Spartacus, and after he refuses to mistreat the young woman, Batiatus takes her away as punishment for not acting as "a man." Over the next weeks, Spartacus excels at gladiatorial skills and falls further in love with Varinia. Marcellus attempts to derail their attraction, but the couple manages to exchange furtive touches. One day, Marcus Licinius Crassus, a patrician in competition with the plebeian Gracchus for control of the Roman Senate, arrives at Capua along with his wife Lady Helena, sister-in-law Claudia and her fiancé, Marcus Glabrus. To celebrate the betrothal, Crassus insists that a gladiatorial match be arranged, ignoring Batiatus' concern that forcing the slaves to fight to the death in their own camp could cause an uprising. Helena and Claudia choose four slaves, including Spartacus and Draba, to fight, and order them to be scantily clad. As the matches begin, the patricians banter happily, undisturbed by the desperation of the fighting men. Spartacus listens from the holding cell as a friend is killed, then enters into battle against Draba. Draba overcomes Spartacus, but, unwilling to kill his compatriot, instead attacks Crassus and is immediately killed by a guard. When Spartacus later hears that Crassus has bought Varinia, he can no longer control his rage, and attacks Marcellus. Emboldened, the other slaves follow suit and escape, forming an "army" that travels across the countryside, looting landowners and freeing slaves, who then join the swelling ranks. Word soon spreads to Rome of the slave rebellion, causing outrage in the senate. While Crassus is away, Gracchus cannily challenges Glabrus, now head of the Roman garrison, to lead some of the troops against the slaves, leaving Julius Caesar as temporary chief of the remaining garrison. When Crassus returns, he comprehends immediately that Gracchus plots to keep Glabrus out of Rome, leaving Crassus more vulnerable to attack. Meanwhile, Spartacus inspires his troops to form a united front that can sweep across the country and escape over the sea to their homelands. In one town, Spartacus is elated to find Varinia, who has escaped and now confesses her love. Back in Rome, while Crassus admires his new "body slave," Antoninus, Gracchus schemes with Batiatus, who blames Crassus for Spartacus' rebellion. Soon, Spartacus' army settles at Mt. Vesuvius, where an escaped Antoninus impresses Spartacus, who longs for an education, with his songs. One day, Tigranes, a representative of Salician pirates, visits to offer the slaves support. Spartacus trades the army's riches for 500 ships, to await the army on the east coast of Italy. Tigranes agrees to the trade, and when he wonders aloud why Spartacus believes he can defeat the mighty Roman garrison, the former slave replies that, unlike soldiers, his men are not afraid to die, since even death is preferable to a life in chains. Soon after, Glabrus arrives and, underestimating the intelligence of the slaves, fails to prepare his troops adequately. Spartacus is able to destroy the garrison and capture Glabrus, whom he sends back to the senate with the message that the army will not be stopped. Crassus is forced to banish Glabrus and retire in shame. Throughout the winter, Spartacus' ever-growing group crosses the country, many dying along the way. In the spring, Spartacus is overjoyed to learn that Varinia is pregnant. Meanwhile, Gracchus convinces the senate to name Caesar as commander of the garrison and to send two legions to destroy Spartacus. When no one volunteers to lead the legions against Spartacus, Gracchus is forced to ask Crassus, who is delighted to head the campaign to "restore order" to Rome. Later, Gracchus reveals to Caesar that he has maneuvered the sale of the Salician ships to Spartacus, knowing that Spartacus' triumph will spell defeat for Crassus. Although Spartacus celebrates upon reaching an encampment a mere twenty miles away from the Salician ships, Tigranes soon arrives, with the news that Pompey and Crassus have conspired to surround Spartacus' army, necessitating the withdrawal of the ships. Spartacus realizes that Crassus is forcing him to attack Rome, which will allow the patrician to use all the troops at his disposal against them. Dismissing Tigranes' offer to smuggle Spartacus and Antoninus, now his closest aide, to freedom, Spartacus instead stirs his troops to march against Rome. At the same time, the Romans elect Crassus as head consul and leader of the legions, and he vows to destroy Spartacus and restore order to the empire. The armies soon come within fighting distance of each other, and Crassus, single-minded in his fear of and hatred for Spartacus, pays Batiatus to identify the former slave on the battleground. Just before the battle, Spartacus tells Varinia that his only prayer is for his son to be born free and to learn about his father's cause. Within hours, Crassus' trained troops have overcome the slave army, and Crassus announces to the survivors that they will be spared crucifixion if they identify Spartacus. Spartacus stands to speak, but before he can sacrifice himself, Antoninus stands and declares, "I am Spartacus." One by one, each slave follows suit, choosing death over betraying the man who brought him freedom. Enraged, Crassus orders them all to be crucified during a long march, lining the road to Rome with their bodies. He also finds Varinia, clutching Spartacus' newborn son, and sends her to his estate. Along the march, Crassus recognizes Antoninus and then, upon spotting Spartacus, guesses he may be his enemy, and orders the two men be kept alive until they reach his estate. There, he banishes Gracchus to the country, intending to use him in the future for his popularity with the "rabble." Soon after, Batiatus experiences what Gracchus terms "a bad case of dignity" and refuses to identify Spartacus, and instead plots with Gracchus to steal Varinia from the estate in order to irritate Crassus. Crassus dotes on Varinia, whose love he believes will prove his superiority over Spartacus but she vows never to stop loving Spartacus. Meanwhile, Spartacus mourns Varinia and his son, who he assumes have died. When Crassus confronts Spartacus, the slave spits in his face, spurring the dictator to order him to fight Antoninus to death, with the winner to be crucified. Spartacus and Antoninus fight valiantly, each trying to save the other from a more painful death, and Spartacus soon triumphs. After murmuring that he loved Spartacus like a father, Antoninus dies, and Spartacus proclaims that "he will come back, and he will be millions." Crassus, fearful even in his victory, orders Spartacus crucified at the gates to Rome. Meanwhile, Batiatus brings Varinia and the boy to Gracchus, who presents them with falsified papers that will allow them freedom, then kills himself. As Varinia leaves Rome, she catches sight of Spartacus on the cross. In his last moments of life, Spartacus sees Varinia lift his son and hears her declare that the boy, now free, will never forget his father.