In Julius Caesar, how do the plebians (people) betrayal of Brutus help develop the plot?
In Act III, Scene 1 of Julius Caesar, Brutus promises Marc Antony that when he goes to the pulpit to provide the reasons for Caesar's death, he will tell the Romans that Antony "speaks by leave and permission" to them afterward. The implications of this statement of Brutus, that Antony is essentially in accord with the conspirators, certainly conflict with the purposes of Antony to turn the crowd against the conspirators. So, when the Romans hear him, they become more and more perplexed that Brutus should give his permission for Antony to stand before them and make insinuations against the honorable intentions of the conspirators. Then, when Antony produces the proof of Caesar's love of the people, Caesar's will which gives each citizen money--a will Antony says he will not read because it will inflame them--the Romans begin to truly question the motives of Brutus and the others. Further, Antony's rhetoric stirs the crowd emotionally so much that one plebian shouts "They were traitors!" and another, "They were villains, murderers!" Finally, all shout,
Revenge! About! Seek! Burn! Fire! Kill! Slay! Let not a traitor live! (3.2.215)
At this point, the crowd becomes an irrational mob, and Antony's prophecy of his soliloquy over Caesar's body becomes true,
A curse shall light upon the limbs of men;
Domestic fury and fierce civil strife
Shall cumber all the parts of Italy;
Blood and destruction shall be so in use,(3.1.282-285)
The civil war that thus ensues after the plebians turn against Brutus and the conspirators, of course, creates the sequence of events that compose Acts IV and V as Antony joins with Lepidus and Octavius Caesar, the adopted son of Julius Caesar to battle with Brutus and Cassius's armies in order to restore the republic ruled by the Caesars and rid Rome of all those who have conspired against Julius Caesar.
The people or plebeians betray Brutus and a civil war breaks out. Antony convinces the plebeians that the honorable Brutus has not acted so honorably. The plot thickens here. The rising action is that Brutus and the conspirators are fleeing for their very lives.
Although the people have always believed Brutus was a man to be respected, Antony stirs up the people's emotions and they turn against Brutus. This betrayal develops the plot in that a the people unite to fight Brutus and his forces.
Now that a civil war is raging, Brutus and Cassius are losing battles. In utter hopelessness, Cassius and Brutus fall on their own swords. This is the climax of the story for the battle has ended. Antony, Octavian, and, in a sense, Caesar win the war. Brutus has fallen and Rome will never be the same.