The play opens with Julius Caesar’s triumphal entry into Rome after defeating his rival, Pompey. It’s also the feast of Lupercal, an annual Roman holiday. During the festivities, a soothsayer warns Caesar to “Beware the idea of March”—an omen Caesar quickly dismisses. Meanwhile, Cassius tries to persuade Brutus that Caesar is dangerously ambitious. Brutus admits that although he loves Caesar, he doesn’t want Caesar to become king, and he desires the good of Rome above all else. After the Lupercal race, Casca informs them that Antony offered Caesar a crown three times, and Caesar refused it each time, although he thinks that Caesar looked increasingly reluctant to say no with each refusal. Brutus and Cassius agree to talk later; Cassius plots to leave Brutus fake letters denouncing Caesar’s ambition. Later, the streets of Rome are filled with fearful omens like meteors, earthquakes, lions, and owls. Cassius persuades Casca that the omens are signs of Caesar’s imminent tyranny, winning him over to the conspiracy.
Brutus, who hasn’t slept since Cassius spoke to him of the conspiracy, talks himself into believing that assassinating Caesar is the best thing to do for Rome. He reads an anonymous letter urging him to “redress” unspoken injustices against Rome. Then Cassius arrives with the other conspirators. Brutus continues to be motivated more by principle than by ambition, emerging as a leader of the plot—he rejects the inclusion of Cicero and the assassination of Antony, who’s close to Caesar. After the conspirators part ways, Brutus’s wife, Portia, begs Brutus to tell her what’s going on. She says that as his wife and Cato’s daughter, she’s stronger than most women; in fact, she’s wounded herself in the thigh to demonstrate her trustworthiness. Brutus promises to confide in her later.
Meanwhile, Caesar can’t sleep, either. His wife, Calpurnia, has dreamed of Caesar’s murder and begs him to stay home from the Capitol that day. In spite of this warning and a fearful augury, Caesar, needing to project invulnerability, determines to go anyway. When conspirator Decius arrives, he confirms Caesar’s decision by reinterpreting Calpurnia’s bloody dream in a favorable light and telling Caesar he’ll be crowned by the Senate that day.
At the Senate, the crowds are chaotic, with various attempts either to warn Caesar or encourage the conspirators. Inside the Capitol, the conspirators kneel before Caesar, presenting the case of Metellus Cimber’s brother’s banishment. They use this opportunity to stab Caesar to death in turn. Caesar dies after saying, “You, too, Brutus?” The conspirators dip their hands and weapons in Caesar’s blood as a sign of Rome’s newfound “liberty” from tyranny. Antony comes in, ostensibly makes peace with the conspirators, and gains Brutus’s permission to speak at Caesar’s funeral, despite Cassius’s suspicions. After the others leave, Antony prophesies that brutal civil war will break out, prompted by Caesar’s vengeful spirit, and he begins to plot with Caesar’s nephew and heir, Octavius Caesar.
At Caesar’s funeral, Brutus appeals to the people’s reason, arguing that it was necessary to kill Caesar for the sake of Rome. At first, they shout their approval. When Antony addresses the people, however, he gives an emotionally charged speech, grieving Caesar and ironically praising Brutus as “honorable.” By the time he concludes his speech with a reading of Caesar’s will, he has incited the people to vengeful rebellion, and Brutus and Cassius have fled the city.
Later, Antony, Octavius, and Lepidus—the Second Triumvirate—make battle plans and consider whether to execute other conspirators. In their own army camp, Brutus and Cassius have a disagreement, centered on Brutus’s stubborn sense of principle and Cassius’s pragmatism. After the argument descends into an exchange of insults and Cassius’s dramatic demand that Brutus kill him, the two reconcile. Brutus explains he is short-tempered because of the news of Portia’s recent suicide—she killed herself by eating hot coals because she feared that Brutus couldn’t defeat Antony and Octavius. Brutus successfully argues that their army should go on the offensive, marching to confront Antony and Octavius’s troops at Philippi. That night, Caesar’s ghost appears to Brutus for the first time, warning him that they’ll meet again at Philippi.
On the battlefield, the leaders exchange taunts. Later, Brutus and Cassius bid farewell to each another—Cassius newly wary of omens, and Brutus asserting that his Stoic beliefs will keep him from suicide, no matter what happens. As the battle rages, Brutus prematurely orders Cassius’s forces to attack Octavius’s vulnerable ones, giving Antony’s forces a chance to overrun them. Influenced by omens and a hasty assessment of the battle’s outcome, Cassius commits suicide. When he learns of this, Brutus believes that Caesar’s Ghost is getting revenge on the conspirators’ wrongdoing, but he commits to seeing the battle through.
Later, on the run from the enemy, Brutus finally falls on his own sword, believing it’s justice for his killing of Caesar. When Antony finds his body, he declares Brutus to have been “the noblest Roman of them all” and arranges for his honorable burial before going off to celebrate his army’s victory.