Caesar has returned in triumph from the war against Pompey, though tribunes Marullus and Flavius express their discontent to the people at the outcome. During the celebrations, a soothsayer warns Caesar to beware the Ides of March.
Cassius and Brutus discuss their fears that Caesar's ambition is to become king, despite learning from Casca that he had refused the people's offer of the crown. Casca, Cassius, and Cinna meet during a stormy night that has been full of unnatural events, and agree the need to win Brutus over to their cause, the elimination of Caesar.
Brutus, meanwhile, has been reflecting on the dangers of having Caesar crowned, so when the conspirators arrive at his house he agrees to the assassination plot. He is reluctant to reveal his intentions to his wife, Portia, but yields to her persuasion. Caesar's wife Calphurnia, frightened by dreams and omens, tries to persuade him not to go to the Capitol. He agrees, but Decius reinterprets the omens in a favourable light, and he leaves, disregarding a warning on the way from Artemidorus.
When Caesar refuses to grant Metellus Cimber's suit, the conspirators kill him. Caesar's friend Mark Antony meets the conspirators, who allow him to speak at Caesar's funeral following Brutus' own speech. Brutus justifies the conspirators' action to the citizens, and receives their support; but Antony's speech rouses them against the conspirators, and Brutus and Cassius flee. The angry people kill Cinna the poet, mistaking him for Cinna the conspirator.
Antony forms a triumvirate with Octavius Caesar and Lepidus, and they plan the deaths of the conspirators, forming an army. Brutus and Cassius join forces at Sardis, and after a fierce personal quarrel agree to fight together at Philippi. Messala brings news of happenings at Rome, and reports that Portia has killed herself.
Caesar's ghost visits Brutus at night, and warns him that he will meet him at Philippi. The two sides parley, then the battle begins.
Thinking they are defeated, Cassius orders his servant Pindarus to kill him. He does so, but it is then revealed that the news was misleading, as Brutus had gained an advantage.
A further battle leads to Brutus' defeat, and he too commits suicide. Antony and Octavius acknowledge Brutus' nobility, and arrange to bury him with honour.