In the first century BC, the Roman Republic has slid into corruption, its menial work done by slaves. One slave, a Thracian named Spartacus, is so uncooperative in his position in a mining pit that he is sentenced to death by starvation. By chance, he is displayed to Roman businessman Lentulus Batiatus, who – impressed by his ferocity – purchases Spartacus for his gladiatorial school and instructs trainer Marcellus to not overdo his indoctrination, believing the slave "has quality". Amid the abuse, Spartacus forms a quiet relationship with Varinia, a serving woman whom he refuses to rape when she is sent to "entertain" him in his cell. The two are forced to endure numerous humiliations for defying the conditions of servitude.
Batiatus receives a visit from the wealthy Roman senator Marcus Licinius Crassus, who aims to become dictator of Rome. Crassus eventually buys Varinia, and for the amusement of his companions arranges for Spartacus and others to fight to the death. When Spartacus is disarmed, his opponent, an Ethiopian named Draba, spares his life in a burst of defiance and attacks the Roman audience, only to be speared in the back by a guard and killed by Crassus. The next day, with the ludus' atmosphere still tense over this episode, Batiatus sends Varinia away to Crassus' house in Rome. Spartacus kills Marcellus, who was taunting him over his affections, and their fight escalates into a riot. Batiatus flees while the gladiators overwhelm their guards and escape into the countryside.
Spartacus is elected chief of the fugitives and decides to lead them out of Italy and back to their homes. They plunder country estates as they go, collecting enough money to buy sea transport from Rome's foes, the pirates of Cilicia. Many slaves join the group, making it as large as an army. Among the new arrivals is Varinia, who escaped while being delivered to Crassus. Another is Antoninus, a slave entertainer who also fled Crassus' service after finding out Crassus expected Antoninus to become his sex slave. Spartacus feels inadequate because of his lack of education. However, he proves an excellent leader and organizes his diverse followers into a tough and self-sufficient community. Varinia, now his informal wife, becomes pregnant.
The Roman Senate becomes increasingly alarmed as Spartacus defeats every army sent against him. Crassus' opponent Gracchus knows that his rival will try to use the crisis as a justification for seizing control of the Roman army. To try to prevent this, Gracchus channels as much military power as possible into the hands of his own protégé, the young senator Julius Caesar. Although Caesar lacks Crassus' contempt for the lower classes of Rome, he mistakes the man's rigid outlook for a patrician. Thus, when Gracchus reveals that he has bribed the Cilicians to get Spartacus out of Italy and rid Rome of the slave army, Caesar regards such tactics as beneath him and goes over to Crassus.
Crassus uses a bribe of his own to make the pirates abandon Spartacus and has the Roman army secretly force the rebels away from the coastline towards Rome. Amid panic that Spartacus means to sack the city, the Senate gives Crassus absolute power. Now surrounded by Roman legions, Spartacus persuades his men to die fighting. Just by rebelling and proving themselves human, he says that they have struck a blow against slavery. In the ensuing battle, most of the slave army is massacred. The Romans try to locate the rebel leader for special punishment by offering a pardon (and return to enslavement) if the men will identify Spartacus. Every surviving man responds by shouting "I'm Spartacus!". Crassus has them all sentenced to death by crucifixion along the Via Appia, where the revolt began.
Meanwhile, after finding Varinia and Spartacus' newborn son, Crassus takes them prisoner. He is disturbed by the idea that Spartacus can command more love and loyalty than he can, and hopes to compensate by making Varinia as devoted to him as she was to her former husband. When she rejects him, he furiously seeks out Spartacus (whom he recognizes from having watched him at Batiatus' school) and forces him to fight Antoninus to the death. The survivor is to be crucified, along with all the other slaves. Spartacus kills Antoninus to spare him this terrible fate. The incident leaves Crassus worried about Spartacus' potential to live in legend as a martyr. In other matters, he is also worried about Caesar, who he senses will someday eclipse him.
Gracchus, having seen Rome fall into tyranny, commits suicide. Before doing so, he bribes his friend Batiatus to rescue Spartacus' family from Crassus and carry them away to freedom. On the way out of Rome, the group passes under Spartacus' cross. Varinia comforts him in his dying moments by showing him his son, who will grow up free and knowing who his father was.
