Rome is in a state of political upheaval. It has long been a limited democracy, but Julius Caesar has just defeated his co-ruler Pompey in a civil war, and assumed sole control. It is unclear whether he plans to preserve democracy, or dissolve the republic and rule as a king.
As Caesar goes to appear before the people, a Soothsayer warns him to "beware the ides of March," but he disregards the prophecy. After Caesar's procession moves on, Cassius draws aside Brutus to discuss Caesar's growing power. Cassius thinks it's unfair that Caesar should rule, since he's no better than they. Brutus admires Caesar, but is strongly opposed to Rome having a king. Casca arrives, and tells them that Antony has just attempted to crown Caesar, and that Caesar refused, but reluctantly. The three men agree to discuss this further. They have been hinting that it may be necessary to assassinate Caesar, but no-one has said this openly. Cassius plans to forge letters to convince Brutus that many Romans fear Caesar, and depend on Brutus to do something.
That night, there is a storm and many people witness supernatural occurrences taken to be bad omens. Cassius and Casca meet others who oppose Caesar, and go to Brutus's house to convince him to join them. Brutus has been awake all night, fearing what Caesar might become. Influenced by Cassius's letters, he decides that Caesar must die. The others arrive, and they all agree to stab Caesar the next morning as he enters the Capitol. Cassius wants to kill Antony too, but Brutus refuses. It now appears that Brutus, not Cassius, is in charge of the plan. When the others leave, Brutus's wife Portia begs him to reveal what's on his mind, saying that he dishonors her by keeping secrets. She has stabbed herself in the thigh to prove herself stronger than other women, and worthy of his trust. Brutus agrees to tell her everything.
The next morning, Caesar is urged to stay at home by his wife Calpurnia, who has seen bad omens and dreamt about his statue spurting blood. Caesar laughs off her concerns, thinking himself invincible, even godlike. When Calpurnia begs him on her knees to stay, he consents, but is convinced again to go when Decius, one of the conspirators, says that the senators will laugh at Caesar for listening to his wife. At the Capitol, the conspirators stab Caesar. When Caesar sees that even Brutus, whom he loved, attacks him, he says "Et tu Bruté?—Then fall Caesar," and dies. There is a panic, and the conspirators don't know whether the people will support or oppose them. Antony arrives, and pretends to make peace with the conspirators, who agree to let him address the people after Brutus. Brutus gives a short speech explaining his reasons for killing Caesar, which satisfies the people, and then leaves. Antony, although claiming he has "come to bury Caesar, not to praise him," stirs up the people by reminding them of Caesar's greatness. A mob forms to hunt down the conspirators, and Brutus and Cassius flee Rome.
Two armies now battle for supremacy—that of Brutus and Cassius on one side, and that of Antony and Octavius, Caesar's nephew, on the other. Brutus and Cassius have begun to argue with each other: Brutus sees Cassius as devious and corrupt, and Cassius sees Brutus as high-minded and overly critical. They make up when Brutus reveals that Portia has killed herself, fearing his defeat. The night before the two armies engage, Brutus sees Caesar's Ghost, who tells him that he will appear again at Philippi, the site of the battle. The battle is essentially a tie, with Brutus's troops defeating Octavius's, and Antony's defeating Cassius's, but Cassius thinks all has been lost and kills himself rather than be captured. When fighting resumes, Brutus's army is finally defeated and he too commits suicide. The victorious Antony expresses admiration over the body of Brutus, who killed Caesar for the good of Rome rather than out of jealousy, calling him "the noblest Roman of them all."