Caesar has just returned from defeating the sons of his former co-ruler, Pompey. The townspeople are celebrating his victory, much to the dismay of various conspirators who dislike Caesar and cringe at the thought of his gaining too much power. As Caesar celebrates at a festival, a soothsayer tells him to beware the Ides of March. Caesar is unconcerned.
Out of Caesar's earshot, a conspirator named Cassius tries to convince Caesar’s friend, Brutus, that he (Brutus) would be a better leader of Rome than Caesar. Cassius talks about Caesar’s weaknesses and urges Brutus to consider how they might get Caesar out of the way. Cassius and Brutus learn that the citizens have tried to crown Caesar king but that he refused the crown three times before having a strange seizure. Cassius sends another insurgent, Cinna, to plant fake letters of praise about Brutus where Brutus will find them. He wants Brutus to think the people adore him so Brutus will embrace the plot to kill Caesar.
Brutus wanders in his orchard at night, still pondering whether he should be part of the plot. Though he’s seen no reason to believe Caesar would let power go to his head, Brutus convinces himself it would eventually happen. His servant Lucius finds one of the letters Cassius has planted, and he brings it to Brutus to read. Cassius and other conspirators arrive, covering their faces. They discuss the plot to kill Caesar on the following day, March 15 (the Ides of March). They also suggest Brutus should kill Caesar’s young friend, Mark Anthony, but Brutus thinks that much bloodshed is unnecessary.
Cassius notes that Caesar may not show up for work the next day because he has become suspicious. A conspirator named Decius says he can flatter Caesar and convince him to come to the Capitol. After the men leave, Brutus’ wife, Portia, comes outside. She knows her husband has had many sleepless nights recently and begs him to tell her what’s really going on. He promises he will, but he urges her to go inside when he has a visit from another conspirator.
Caesar is also wandering around his house, unable to sleep. His wife, Calphurnia, has been calling out in her sleep about Caesar being murdered. She begs him not to go to the Capitol that day. While he dislikes appearing cowardly, he finally agrees to humor her. When Decius comes to Caesar's home, Caesar tells the man he’s staying home. He tells Decius about Calphurnia’s bloody dreams, but Decius quickly invents an alternative interpretation of them. He says the dreams prove Caesar was meant to rule. He also suggests that if Caesar seems cowardly and doesn’t show up, the senate might not give him the crown as they’d planned. Caesar decides it was foolish to listen to his wife, and he goes to the Capitol with Brutus and the other conspirators, who have come to escort him.
On the road near the Capitol, a soothsayer tries to get Caesar to read his note. He has penned a warning letter, telling Caesar his friends are traitors. With his men all around him, Caesar tries to make it look like Rome’s affairs, not personal notes, come first for him. Thus, he doesn’t read the soothsayer’s note and misses the warning about the plot against him.
Caesar makes a big, self-aggrandizing speech during which he denies the pleas of a man who is trying to save his brother’s life. The conspirators strike at Caesar and stab him. As Caesar dies, he utters the famous literary line, “Et Tu, Brute?” meaning he is shocked to learn that even his friend Brutus wants him dead.
Brutus and the conspirators try to get a handle on the situation as the Romans cry out in the streets over Caesar’s death. Brutus and Cassius urge all the conspirators to put Caesar’s blood on themselves so they can march through the streets declaring victory for Rome. Just then, Mark Anthony arrives. Brutus assures Anthony they had good reasons for killing Caesar. Anthony says he trusts Brutus and vows loyalty to him if Brutus will let him speak at Caesar’s funeral. Once he’s alone, Anthony reveals to the audience that he will incite the crowds at the funeral and get them to kill Brutus and the conspirators. Anthony gets word that Caesar’s adopted son, Octavius, is nearing the city. He sends word for Octavius to stay where he is until after Anthony’s funeral speech.
Brutus gives the people a convincing speech about how he loved Caesar but had to kill him because of the ruler’s ambition. The people begin to believe Brutus when he tells them the act was done out of love for Rome. Then Anthony comes to the podium and subtly turns the crowd by convincing them Caesar really did love the people and Rome. When his speech is done and he’s read Caesar’s will to the people, the crowds are ready to burn the conspirators’ houses.
Anthony, Octavius and another man named Lepidus decide they will now rule Rome together. They begin making a checklist of whom they should kill for conspiracy. Once Lepidus leaves, Anthony starts talking about the man’s weaknesses. He convinces Octavius that they should make Lepidus do the bulk of the ruling while they sit back and tell him what needs to be done.
Meanwhile, Brutus and Cassius have left Rome and built armies for themselves. They begin to dislike and distrust each other, and fight privately. They finally cool off and decide to be allies again. Brutus confesses he is feeling grief at the moment because he’s learned Portia killed herself when he fled the city. Cassius and Brutus plan to send their armies to Philippi to meet Octavius and Anthony in battle. That night, Caesar’s ghost visits Brutus and tells him he’ll see him in Philippi.
The next day, Brutus and Cassius meet Anthony and Octavius for a pre-battle powwow. Brutus hopes to negotiate, but Anthony and Octavius refuse. Cassius talks about a bad omen he saw on the way, and he and Brutus discuss whether or not they would kill themselves rather than surrender.
When Cassius is convinced he’s lost the battle, he urges another soldier to kill him. Soon after this, members of the army come to announce a moment of victory. Cassius’ friend and fellow soldier Titinius kills himself with Cassius’ sword when he sees that his friend is dead. Brutus gets the remaining army together. Brutus’ friend Lucilius is captured when he pretends to be Brutus. Anthony knows the man isn’t Brutus, but he keeps Lucilius safely imprisoned in hopes of befriending him.
As the battle turns badly for Brutus and his army, he convinces another soldier to hold his sword while he (Brutus) runs into it. Brutus feels he’s made things right with Caesar. Anthony and Octavius say they will bury Brutus as an honorable man.