General Maximus (RUSSELL CROWE) is one of the greatest military leaders ancient Rome has ever known. Victorious over the last army of Germanic renegades, Maximus looks forward to returning home to his wife and son and living the peaceful life of an everyday farmer.
Yet, the dying Emperor, Marcus Aurelius (RICHARD HARRIS), who is distraught over what Rome has become, wants to hand over his power to Maximus under the condition that he then turn it over to the people through the now subservient Roman senate. Maximus isn't thrilled with the idea for he has no experience in politics, but before he can give the emperor an answer, Aurelius' son, Commodus (JOAQUIN PHOENIX), the biological heir to the throne, covertly kills his father and then asks for the general's support.
When Maximus doesn't give it, the new Emperor orders him and his family to be executed. While Maximus manages to escape, his family doesn't. Shocked from that eventual discovery and ill from a wound he received during his escape, Maximus passes out, only to find that he's been captured and to be sold as a slave.
Purchased along with fellow slave Juba (DJIMON HOUNSOU) by former gladiator, turned promoter, Proximo (OLIVER REED), and posing as a Spaniard, Maximus suddenly finds himself competing in small gladiator contests outside Rome where he must kill or risk being killed. As his reputation grows, Maximus soon discovers that he may have a way to strike revenge on Commodus.
He's returned to the capital with his sister, Lucilla (CONNIE NIELSEN), who's happy to see her young son, Lucius (SPENCER TREAT CLARK), but isn't thrilled about her corrupt brother being her people's ruler, which also holds true for Senator Gracchus (DEREK JACOBI) and other members of the Senate. As Maximus gets ever closer to once again facing Commodus, who's becoming increasingly dangerous and corrupt, it's only a matter of time before events come to a head between the two.