Marcellus and Flavius criticize the commoners for celebrating Caesar's recent military defeat of Pompey since they feel it's actually a sad day. During a victory march, a soothsayer warns Caesar to "Beware the Ides of March" (March 15); Caesar ignores him. A race is run, wherein Marc Antony, in the course of competing, touches Caesar's wife Calphurnia in hopes of curing her infertility. During the race, Cassius tries to convince Brutus that Caesar has become too powerful and too popular. Brutus neither agrees nor disagrees. Caesar confers with Antony that he fears Cassius is evil and worth fearing. Casca explains to Brutus and Cassius that shouting they heard was caused by Caesar's thrice refusal of a crown offered to him by Antony (though confusing, the commoners rejoiced that he had refused it for it indicated he is a noble man). At the third offering, Caesar collapsed and foamed at the mouth from epilepsy. Afterward, Caesar exiled/executed Flavius and Marcellus for pulling scarves off of Caesar's images (statues). In a thunderstorm, Casca meets Cicero and tells him of many ominous and fearful sights, mostly of burning images, he has seen. Cassius then meets Cicero and tells him the storm is a good sign of the evil he and his other cohorts plan to do to Caesar. It seems the senators plan to crown Caesar King, but Cassius aims to prevent it, or else commit suicide. Casca agrees to help Cassius. Cinna informs Cassius that Decius Brutus (actually Decimus), Trebonius, and Metallus Cimber will help them to kill Caesar.
Cassius is trying to convince Brutus to join too. Brutus, unable to sleep, tells himself that he fears Caesar will become a tyrant if crowned king. Cassius et al. come to Brutus and resolve to murder Caesar the next day (March 15). Metallus also convinces Caius Ligarius to join their cause. The men leave and Portia (Brutus' wife) begs Brutus to tell her what is happening, but he does not (though he does tell her before he leaves for the Senate). At Caesar's house, Calphurnia begs Caesar to stay home for fear of danger (based on a foreboding dream and the night's storm). Holy priests pluck the entrails of an animal and find no heart in it, another bad sign. Caesar declares he will stay home, to calm his wive's fears. Decius, though, convinces Caesar to come to the senate. On the way, the soothsayer Artemidorus tries to warn Caesar of impending death, to no avail. At the Senate, Trebonius leads Antony away from Caesar, then the conspirators murder Caesar. They cover themselves in his blood and go to the streets crying, "Peace, freedom, and liberty." Antony comes back and mourns Caesar's murder. Antony pretends to support the clan, yet yearns for great havoc to occur as a result of the death. Brutus explains to the crowd that they killed Caesar because he was too ambitious. Antony replies with reverse psychology to incite the commoners to riot in grief over Caesar's murder. Antony also reads them Caesar's (supposed) will, wherein he leaves money to all the citizens, plus his private gardens. In the ensuing riots, Cinna the poet is wrongly killed by a mob that believes him to be Cinna the conspirator.
Antony forms a triumvirate with Octavius Caesar and Lepidus, to rule Rome. However, Brutus and Cassius are raising an army to defy them. Brutus learns that his wife Portia kills herself by swallowing hot coals. Messala tells Brutus that the triumvirate has killed 100 senators. Titinius, Messala, Brutus, and Cassius decide to confront Antony's army at Phillipi. At Brutus' tent, the ghost of Caesar comes and tells Brutus he will see him at Phillipi. The battle indeed ensues at Phillipi. Cassius confers to Messala that it is his birthday and that he fears defeat. In battle, Titinius is captured by Octavius. Cassius convinces Pindarus to help him commit suicide. Pindarus, in grief, flees after the deed is done. In a twist, Brutus overthrows Octavius and Cassius' army, defeating part of Antony's army. Titinius, in grief over Cassius' death, kills himself with Cassius's sword. The battle turns again, this time against Brutus' army. Cato is killed and Lucilius is captured, while pretending to be Brutus. Brutus successively asks Clitus, Dardanius, and Volumnius to help him commit suicide, yet all refuse. Brutus finally convinces Strato to hold the sword while he (Brutus) runs onto it and dies. Thus, Antony and Octavius prevail, while Cassius and Brutus both commit suicide, assumedly partly in grief over murdering Caesar.