In the 1st century BC, the Roman Republic has slid into corruption, its menial work done by armies of slaves. One of these, a proud and gifted Thracian named Spartacus (Kirk Douglas), is so uncooperative in his position in a mining pit that he is sentenced to death by starvation. By chance, he is displayed to unctuous Roman businessman Lentulus Batiatus (Peter Ustinov), who – impressed by his ferocity – purchases Spartacus for his gladiatorial school, where he instructs trainer Marcellus (Charles McGraw) to not overdo his indoctrination because he thinks “he has quality”. Amid the abuse, Spartacus forms a quiet relationship with a serving woman named Varinia (Jean Simmons), whom he refuses to rape when she is sent to “entertain” him in his cell. Spartacus and Varinia are subsequently forced to endure numerous humiliations for defying the conditions of servitude.
Batiatus receives a visit from the immensely wealthy Roman senator Marcus Licinius Crassus (Laurence Olivier), who aims to become dictator of the stagnant republic. Crassus buys Varinia on a whim, and for the amusement of his companions arranges for Spartacus and three others to fight in pairs. When Spartacus is disarmed, his opponent, an African named Draba (Woody Strode), spares his life in a burst of defiance and attacks the Roman audience, but is killed by an arena guard and Crassus. The next day, with the ludus’ atmosphere still tense over this episode, Batiatus takes Varinia away to Crassus’s house in Rome. Spartacus kills Marcellus, who was taunting him over his affections, and their fight escalates into a riot. The gladiators overwhelm their guards and escape into the Italian countryside.
Spartacus is elected chief of the fugitives and decides to lead them out of Italy and back to their homes. They plunder Roman country estates as they go, collecting enough money to buy sea transport from Rome’s foes, the pirates of Cilicia. Countless other slaves join the group, making it as large as an army. One of the new arrivals is Varinia, who escaped while being delivered to Crassus. Another is a slave entertainer named Antoninus (Tony Curtis), who also fled Crassus’s service. Privately, Spartacus feels mentally inadequate because of his lack of education during years of servitude. 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 by him, and he also comes to regard the spirited Antoninus as a sort of son.
The Roman Senate becomes increasingly alarmed as Spartacus defeats the multiple armies it sends against him. Crassus’s populist opponent Gracchus (Charles Laughton) knows that his rival will try to use the crisis as a justification for seizing control of the Roman army. To try and prevent this, Gracchus channels as much military power as possible into the hands of his own protege, a young senator named Julius Caesar (John Gavin). Although Caesar lacks Crassus’s contempt for the lower classes of Rome, he mistakes the man’s rigid outlook for nobility. 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 Romans, Spartacus convinces 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, after initially breaking the ranks of Crassus’s legions, the slave army ends up trapped between Crassus and two other forces advancing from behind, and most of them are massacred. Afterward, 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, living or dead. Every surviving man responds by shouting “I’m Spartacus!”. As a result, Crassus has them all sentenced to death by crucifixion along the Via Appia between Rome and Capua, where the revolt began.
Meanwhile, Crassus has found Varinia and Spartacus’s newborn son and has taken 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 men captured after the great battle. Spartacus kills Antoninus to spare him this terrible fate. The incident leaves Crassus worried about Spartacus’s potential to live in legend as a martyr. In other matters, he is also worried about Caesar, whom 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’s family from Crassus and carry them away to freedom. On the way out of Rome, the group passes under Spartacus’s cross. Varinia is able to comfort him in his dying moments by showing him his little son, who will grow up free and knowing who his father was.