In Rome, Commodus reopens the gladiatorial games to commemorate his father's death, and Proximo's company of gladiators are hired to participate. In a recreation of the Battle of Zama (incorrectly named the Battle of Carthage) at the Colosseum, Maximus leads Proximo's gladiators to decisive victory against a more powerful force, much to the amazement of the crowd. Commodus descends into the arena to meet the victors and is stunned to discover Maximus as the leader of Proximo's gladiators. The Emperor, unable to kill Maximus because of the crowd's roaring approval for him, sulks out of the arena.
As the games continue, Commodus pits Maximus against Tigris of Gaul, Rome's only undefeated gladiator, in an arena surrounded by chained tigers with handlers instructed to target Maximus. Following an intense battle, Maximus narrowly defeats Tigris and awaits Commodus' decision to kill or spare Tigris. As Commodus votes for death, Maximus spares Tigris, deliberately insulting the Emperor and garnering the audience's approval. His bitter enemy now known as "Maximus the Merciful," Commodus becomes more frustrated at his inability to kill Maximus or stop his ascending popularity while Commodus' own popularity shrinks.
Following the fight, Maximus meets his former servant Cicero, who reveals that Maximus's army remains loyal to him. They are camped at the port Ostia. Maximus forms a plot with Lucilla, Commodus' sister, and Senator Gracchus to reunite Maximus with his army and overthrow Commodus. Commodus however, suspecting his sister's betrayal, threatens her young son and forces her to reveal the plot. Praetorian guards immediately storm Proximo's gladiator barracks, battling the gladiators while Maximus escapes. Hagen and Proximo are killed in the siege while Juba and the survivors are imprisoned. Maximus escapes to the city walls only to witness Cicero's death and be ambushed by a legion of Praetorian guards.
Concluding that legends born in the Colosseum must die there, Commodus personally challenges Maximus to a duel in front of a roaring audience. Acknowledging that Maximus' skill exceeds his own, Commodus deliberately stabs Maximus with a stiletto, puncturing his lung, and has the wound concealed beneath the gladiator's armor. In the arena, the two exchange blows before Maximus rips the sword from Commodus's hands. Commodus requests a sword from his guards, but they betray him and refuse to lend him their weapons. Maximus drops his own sword, but Commodus pulls a hidden stiletto and renews his attack. Maximus then beats Commodus into submission and kills him with his own stilletto.
As Commodus collapses in the now-silent Colosseum, a dying Maximus begins seeing his wife and son in the afterlife. He reaches for them, but is pulled back to reality by the Praetorian prefect Quintus, who asks for instructions. Maximus orders the release of Proximo's gladiators and Senator Gracchus, whom he reinstates and instructs to return Rome to a Senate-based government. Maximus collapses, and Lucilla rushes to his aid. After being reassured that her son is safe and Commodus is dead, Maximus dies and wanders into the afterlife to his family in the distance. Senator Gracchus and Proximo's gladiators carry his body out of the Colosseum. That night, a newly freed Juba buries Maximus' two small statues of his wife and son in the Colosseum, and says that he too will eventually join them, but not yet.