Russell Crowe plays Maximus, the top general in the great Roman Army. He has a wife and 8 year old son that he loves dearly but hasn't seen them for over 2 years because the one thing he loves even more is serving Marcus Aurelius Caesar, (Richard Harris). They are just finishing the last great battle that will finally bring peace throughout the Roman empire. Maximus is loved by his men as he rallies them for the big fight and ultimately is victorious. Marcus Aurelius Caesar is there by his side and he loves him like a son. In fact, loves him more than his own son Commodus, played by Joaquin Phoenix.
Marcus Aurelius Caesar is about to die and in a private conversation with Maximus, tells him that his son is not capable of leading Rome and that Rome is in such poor condition now that it should no longer have an emperor, but instead be a republic with the Senate running it. He wants Maximus to be in charge of the Senate since he is a great man and all of Rome will respect him, unlike his son Commodus.
When Marcus Aurelius Caesar tells Commodus his plan, they embrace and Commodus hugs his father so tightly he suffocates him, killing Marcus Aurelius Caesar and thereby becoming the new emperor of Rome. The first thing he does is to order the death of Maximus and his family.
Maximus is led out to the forest and just before he is put to death, escapes and rushes to save his wife and son. He is to late. When he arrives home his house has been burned down and his wife and son crucified and burned.
Maximus breaks down and is so upset he passes out. When he comes to, he is being picked up by a slave trader and sold to Proximo (Oliver Reed). Proximo buys slaves to become Gladiators. Gladiator fighting has been banished from the city of Rome and he is forced to stage these fights in small villages around the empire. These fights are kind of like cock fighting with humans and Proximo is the man supplying the fighters.
Maximus teaches the rest of the gladiators how to fight and how to fight as a team. Suddenly, he is making a real name for himself as one of the greatest gladiators ever.
Commodus decides to bring back Gladiator fighting to the city because he knows the people like it and he does anything he can to make the citizens like him. Proximo is happy since he is being paid big money to bring his now famous Gladiators to Rome.
During the first fight, Commodus sends out his best soldiers to fight the Gladiators. Maximus rallies his men, much like in battle and defeats the Roman soldiers. The crowd goes nuts. At the end of the fight, Commodus gives a thumbs up which means the gladiators will live to fight another day, all the while knowing that the citizens will love him for granting the masked gladiator (Maximus) life.
Fearing that the crowd is liking the unknown gladiator more then they like him, Commodus stages another fight with the greatest fighter in the Roman Army. Maximus defeats him as well and just before he plunges the sword in him, he waits for Commodus' signal. The crowd is screaming for death to the soldier. Commodus doing whatever makes the crowd happy, gives a thumbs down, signaling the Gladiator to kill him. Maximus, trying to make Commodus look bad, drops his sword and walks away. The crowd goes nuts.
An infuriated Commodus goes down to the arena and asks the Gladiator to reveal himself. Maximus tells him that he is the husband of a murdered wife, father of a murdered son and as he takes off his mask tells Commodus that he is the General of Marcus Aurelius Caesar's great army. The crowd is now going crazy as Maximus is the most popular man in all of Rome. He is also however, still a slave and jailed in the city's prison.
Maximus once loved Commodus' sister Lucilla. She comes down to the dungeon to talk to him. He has a plan to escape from the city, find his old army exiled somewhere in the Roman empire, come back and defeat Commudus in a great battle, then put the best Senator in charge, just as Marcus Aurelius Caesar would have wanted it.
She agrees and puts the plan in action. Unfortunatly, she told her son too much and while he's playing with Commodus, he accidently hints at the plan. That night at dinner, Commodus threatens the boys life if Lucilla doesn't tell Commodus the entire plan. She does and Maximus is captured while trying to escape and most of his fellow gladiators are killed.
Commodus loves his sister very much. Really loves her! He wants to impregnate her so their son will be a full blooded emperor. She resists his advances but knows that he will probably get his way eventually.
Commodus wants the people of Rome to love him but they instead want Maximus. To prove himself to the people, Commodus stages a battle between he and Maximus. He meets Maximus in the jail before the fight and while Maximus is still chained up, Commodus stabs him, nearly killing him. He then puts a fighting suit on Maximus to hide the wounds. They both enter the arena.
Maximus, now near death, defeats and kills Commodus in a great battle in front of a packed colosseum crowd. They are cheering wildly during the fight but when Maximus kills Commodus, the crowd is silent and Maximus tells the Senators and the rest of the crowd, what Marcus Aurelius Caesar's plan was. He falls to the ground and Lucilla runs to him. She tells him to let go and be with his wife and son.
In a dream, he returns home with the two of them waiting for him.
Maximus' body is carried off in a hero's procession while Commodus' body remains ignored in the colosseum dirt.