Julius Caesar returns triumphantly to Rome having defeated his enemy, Pompey. Many Roman senators, however, suspect him of wanting to become Emperor. Their chances of forestalling Caesar rest on being able to convince Marcus Brutus, one of the most upright men in Rome and Caesar's friend, that Caesar is a danger to the state.
As Caesar leads the victory procession through Rome, a Soothsayer tells him to "Beware the Ides of March." Caesar scoffs and continues. After the procession passes, Cassius, who fears Caesar's power and his intentions, begins to recruit Brutus to his conspiracy. Brutus is wary of Cassius' motives, but tells Cassius he will consider the matter, as he has his own suspicions about Caesar's intentions. That evening, during a wild storm, Cassius recruits Casca to the cause against Caesar. Cassius also sends Cinna to throw forged letters, supposedly from concerned Roman citizens, in Brutus' windows.
As Brutus paces the floor, he reads a letter which convinces him to join the insurgency. All the conspirators then appear at his house where they swear common cause and plan an assassination. Brutus argues against killing Mark Antony, known to love Caesar. He prevails, although not all agree with him. After they leave, Brutus' wife Portia convinces him to tell her what is going on, making the point of her own noble lineage as one who is entitled to know and able to cope with the knowledge.
Calpurnia, Caesar's wife and a believer in omens, tries to keep Caesar from going to the Senate that day. First he agrees, then at the urging of Decius, a conspirator, he goes.
At the Senate, after being greeted by the conspirators, Caesar is stabbed by all of them and dies. Antony flees, then returns and pretends to treat the conspirators as liberators. He asks to speak at Caesar's funeral, a boon Brutus grants him against Cassius' advice. At the funeral the crowd acclaims Brutus' words. When he leaves, Antony exhorts the crowd by saying, "Friends, Romans, countrymen…" His ironic tone turns the crowd against the conspirators. The angry mob goes after Brutus and the conspirators, who flee.
Antony meets Octavius Caesar, Julius Caesar's adopted son, and with M. Aemilius Lepidus forms a triumvirate to combat the conspirators. The men decide who among the conspirators must live and who must die.
Brutus, angry with Cassius, verbally attacks him for dishonesty and graft, then when they have made their peace, tells him that Portia has killed herself. A large army led by Antony and Octavius is approaching them. The commanders decide to meet the army at Philippi rather than wait for it to attack. Then, when Brutus is alone with his sleeping servants, the ghost of Caesar visits Brutus and tells him they will meet again at Philippi.
On the field of battle, the two armies parley, but the conversation degenerates into name-calling. While preparing for battle, Cassius tells Messala, one of his generals, that omens suggest they will lose, although he doesn't completely believe it. During the battle, Cassius' servant tells him wrongly that his messenger, Titinius, has been taken, and Cassius kills himself. When Titinius returns in triumph, he finds Cassius' body, and kills himself in grief. Brutus, looking for Cassius' body, finds both of them.
The battle rages on, and Brutus inspires his men to keep fighting. Soon, however, he asks his loyal men to kill him. Each refuses until Strati agrees to hold the sword while Brutus runs upon it. Mark Antony and Octavius find Brutus' body. In tribute, Mark Antony says of Brutus, "He was the noblest Roman of them all."