In 44 BCE the general Julius Caesar returns to Rome victorious after defeating his rival Pompey. Roman citizens cheer for him as he parades through the streets. But he's not welcomed by everyone. Two tribunes, Flavius and Marullus, fear Caesar has become too idolized; they strip Caesar's statues of their ceremonial adornments. As Caesar prepares to greet the public at the Festival of Lupercal, a soothsayer approaches. "Beware the Ides of March [March 15]," he warns. Caesar ignores him.
Caesar's friends Cassius and Brutus discuss the leader's quest for power. Cassius is concerned that Caesar will become a dictator. He fears that the Roman republic would thus be destroyed, taking the freedom of its citizens with it. Brutus shares Cassius's concerns but remains loyal to Caesar.
Cassius concocts an assassination plot to kill Caesar. He enlists Casca, Decius, Trebonius, Ligarius, Metellus Cimber, and eventually Brutus as conspirators. They contemplate killing Antony, a close associate of Caesar, but decide against it. Meanwhile, Caesar's wife, Calphurnia, has an ominous dream that she believes predicts Caesar's death. Caesar dismisses her misgivings and goes to the Senate anyway on the Ides of March.
On the Senate floor, Caesar refuses Metellus Cimber's petition to repatriate a banished brother. On cue the conspirators rise up and attack Caesar. When Brutus strikes, Caesar—shocked at his friend's betrayal—says, "Et tu, Brutè?" ("You too, Brutus?")
Antony discovers the conspirators with Caesar's body. Antony grieves but convinces the conspirators he will take their side in public.
At Caesar's funeral Brutus speaks first. He tells the crowd he killed Caesar out of love for Rome and fear of Caesar's dangerous ambition. The crowd believes Brutus and agrees with him. Antony speaks next. He addresses the crowd as "Friends, Romans, countrymen," gaining their favor, and says that Caesar was a good ruler who died a cruel death. He gives multiple examples of Caesar's lack of dictatorial ambition, unveils Caesar's body, and reads the crowd Caesar's will, which bequeaths money to every citizen. Antony's speech and actions rile the crowd against the conspirators—just as Antony planned. Cassius and Brutus are forced out of the city and build armies to fight Antony's forces.
Octavius Caesar arrives in Rome. Octavius is Julius Caesar's great-nephew and adopted son. He has also been named as his great-uncle's successor. Following the funeral he meets with Antony and the general Lepidus. The three plan to overtake Brutus and Cassius's forces and rule Rome themselves.
Brutus and Cassius quarrel while in camp preparing for battle. Cassius refuses to punish a soldier who accepts bribes; Brutus fears they will all descend into corruption if they don't act honorably, and Caesar's death will be in vain. Eventually the two reconcile. Brutus confesses that his wife, Portia, committed suicide after Brutus's long absence.
Later that night, Brutus receives a visit from Caesar's ghost. The ghost says Brutus will see him again the next day—at Philippi, the site of the battle with Antony, Octavius, and Lepidus's forces.
In Philippi the battle begins. Cassius's forces quickly lose ground. Cassius sends his servant Pindarus to see how Brutus's troops are faring elsewhere on the battlefield. Pindarus brings back bad news—Cassius's friend and fellow soldier Titinius has been captured. Titinius is actually celebrating a victory with his troops, but Pindarus and Cassius are watching from too great a distance to see this. Cassius, in despair, asks Pindarus to kill him.
Brutus continues to fight but loses to the opposition. Antony has called for Brutus's capture, dead or alive. Brutus falls on his sword rather than return to Rome as a slave. He claims (as did Cassius when he died) that Caesar is avenged.
Antony and Octavius, now victors, come upon the fallen Brutus. Antony praises Brutus's nobility and decides to bury him as a war hero.