Flavius and Marullus break up a crowd of commoners celebrating the victory of Julius Caesar, who has returned to Rome from a civil war fought against his co-ruler, Pompey. The two officials shame the people for celebrating the defeat of one Roman by another, and tear down the decorations honoring Caesar. Attending a race in celebration of the Roman Holiday Lupercalia, Caesar asks his friend Marc Antony to touch his wife Calphurnia as he runs by in the race. (Roman tradition held that a woman’s pregnancy was assured by the touch of a runner in the race– and Caesar had no heir.) A Soothsayer approaches Caesar and warns him to “beware the Ides of March”—the fifteenth day of March. Caesar ignores him and continues on to the festival.
Two senators, Cassius and Brutus, stay behind, discussing how they fear that Caesar wants to rule Rome single-handedly. Cassius tells Brutus that the people are looking to him for leadership. They hear cheers, and fear the populace is cheering in support of Caesar as their king. Cassius reminds Brutus of the revolt that founded the Republic—led by Brutus’s own ancestor—in hopes of coaxing Brutus to join the conspirators’ cause against Caesar’s rule. Brutus admits to having a similar plan in mind. Casca, another Roman senator, joins them and reports that Caesar was indeed offered a crown three times, but declined it each time. Left by himself, Cassius reveals that he believes Brutus to be easy to manipulate, and plans to convince him to rise up against Caesar by sending Brutus forged letters, supposedly from angry citizens urging him to take action against Caesar. Cassius convinces Casca to join the conspirators, and they plan to meet later in secret to discuss the plot. Cassius instructs Cinna, a fellow conspirator, to leave the forged letters for Brutus.
Sleepless, Brutus is contemplating joining the conspirators in their plot when his servant, Lucius, brings him one of the forged letters he found. The letter asks Brutus to take leadership for the people of Rome. Lucius announces the arrival of a group of men requesting to speak with Brutus. The conspirators come in the middle of the night to convince Brutus that Caesar must be killed, and that only Brutus can save Rome from tyranny. Brutus agrees that Caesar must die, but convinces the others that to kill Antony would prove them nothing more than bloodthirsty savages. They decide the assassination will happen the next day at the Senate. Decius volunteers to make sure that Caesar shows up. Brutus’s wife Portia asks him why he has been acting so strange and distant from her, and he promises to tell her later. Calphurnia, Caesar’s wife, warns her husband of her premonition in her dreams of his violent death, and begs him to stay home from the Senate. Caesar doesn’t listen, but when she implores him to stay as a favor to her rather than as a bow to caution, he consents. However, when Decius arrives at Caesar’s home to ensure his presence in the Senate that day, he deceptively interprets Calphurnia’s nightmares favorably. He tells Caesar that the senators are planning to offer him the crown that day, and that not showing up may force them to reconsider their decision. Artemidorus, a teacher, plans to intercept Caesar with a letter warning him not to trust the conspirators.
Caesar receives two more warnings on his way to the Senate—from the Soothsayer again as well as the letter from Artemidorus, which he never reads. At the Senate, Trebonius lures Caesar’s devoted friend Marc Antony away as the conspirators surround Caesar and stab him to death. Brutus orders them all to bathe their hands and swords in Caesar’s blood. Upon seeing Caesar dead in cold blood, Marc Antony pledges himself to the conspirators, asking only to be allowed to speak to the public at his friend’s funeral. Brutus grants Antony’s wish against Cassius’s strong objections. Left alone with Caesar’s body, Antony promises revenge. Brutus speaks to the bewildered Romans, explaining that Caesar’s death was necessary to keep Rome in the hands of the people, and the crowd is momentarily appeased. When Antony addresses the people, he claims that Caesar had no intention of taking Rome out of the hands of the people, and succeeds in turning the crowd against the conspirators. The angry mob rages through the streets of Rome, searching for anyone responsible for Caesar’s death. They come upon a poet named Cinna and taking him for Cinna the conspirator, kill him because he shares the name of one of the conspirators. Brutus and Cassius escape the mob and flee the city.
Ruling Rome as a triumvirate, Antony, Octavius (Caesar’s nephew and heir) and their ally Lepidus join forces to prepare for war against Caesar’s murderers. Between them, they coldly barter who will live and who will die, agreeing to kill Lepidus’s own brother.
Hiding out in the city of Sardis, Cassius confronts Brutus in a fury for punishing one of his men for accepting bribes. Brutus in turn accuses Cassius of taking bribes. The fight escalates to the point where Cassius asks Brutus to kill him. Brutus’s rage spent, the two men forgive one another. Brutus confides that Portia has taken her own life. Brutus convinces Cassius not to wait for Antony’s army to find them, but to head to Philippi to seek out their enemy. Alone in his tent, Brutus sees Caesar’s ghost who tells him that they will meet again at Philippi.
Failing at dialogue, the two sides meet again on the battlefield. Fearing that Antony’s soldiers are approaching, Cassius sends Titinius, his scout, to discover their identity. The troops are actually members of Brutus’s army, and Titinius joins their ranks. Pindarus mistakenly reports to his master Cassius that the enemy has captured Titinius. Believing the battle lost, Cassius orders Pindarus to kill him. Brutus attacks the enemy again, but is defeated. Claiming to be Brutus to protect his master, Lucius allows himself to be taken prisoner. Antony realizes that Lucius is not Brutus and spares him. Convinced that all is lost, Brutus urges one of his soldiers to assist him in his suicide lest he be taken prisoner by Antony, and runs himself into his own sword. Antony finds Brutus’s corpse and proclaims him the noblest Roman of all.