Coriolanus tells a group of Roman nobles that he has no intention of changing his character to suit the desires of the mob. Volumnia comes in and berates him for his intransigence, and then Menenius arrives with the Senators and advises him to go the marketplace and make peace with the people: he must recant what he has said about the plebeians and their tribunes, and then perhaps they will allow him to be consul. Coriolanus refuses, preferring to keep his honor, but his mother advises him to act humbly, even if his humility is dishonest, and ask for pardon, even if he does not mean this. He remains obdurate for a long while but eventually relents and agrees to make peace with the plebeians.
In the marketplace, Brutus and Sicinius prepare for Coriolanus's arrival, planning to bait him into losing his temper. The war hero enters, accompanied by Menenius and Cominius, and declares that he will submit to the will of the people. However, when Sicinius accuses him of planning to tyrannize the Roman state, he immediately becomes furious and again launches into a tirade against the tribunes and plebeians. As his friends watch helplessly, Sicinius and Brutus, supported by the entire populace, and over the protests of Cominius, declare that he must be banished from Rome forever. Coriolanus replies that he will go gladly, and he prepares to leave the city, pausing only to bid farewell to his wife, Virgilia, and to his mother and friends. Volumnia weeps and curses the city for casting him out, while Cominius offers to accompany him for a time, but Coriolanus refuses these offers and departs.
Brutus and Sicinius dismiss the people, and then try to avoid encountering Volumnia, Virgilia, and Menenius, who are returning from bidding farewell to Coriolanus. Volumnia spots the two tribunes, however, and denounces them, saying that they have exiled the best man in Rome. Brutus and Sicinius accuse her of having lost her wits, and they depart, leaving the friends of Coriolanus to their grief.
Meanwhile, a Roman in the pay of the Volscians meets up with another Volscian spy and reports that Coriolanus has been banished. The two men agree that this will give Tullus Aufidius an excellent chance to gain some revenge against Rome for the defeats he has suffered. At the same time, Coriolanus himself comes to the city of Antium, where Aufidius is staying. He informs the audience that he plans to ally himself with Aufidius against his native city and become Rome's greatest enemy.
Coriolanus once obeyed his mother by pandering to the masses, but now he questions her: "Why did you wish me milder?" he asks her: "Would you have me false to my nature (III.ii.14-15)?" This is the critical question for him: He is not a reflective hero, and Shakespeare does not give him the space for soliloquy that other heroes enjoy, but Coriolanus knows that he has betrayed some essential part of himself by trying to gain the people's favor, and he wonders why he should bother to make the same betrayal again by making peace with the plebeians. The obvious answer is offered by a Senator, who tells him that "by not so doing, our good city / Cleave in the midst, and perish (III.ii.27-28)." But the stronger reason, and the reason that ultimately persuades him to make one final attempt at compromise, is that voiced by Volumnia. She says, "I prithee now, sweet son, as thou hast said / My praises first made thee a soldier, so, / To have my praise for this, perform a part / Thou hast not done before (III.ii.107-110)." That is, she tells him to do it because she wants him to; just as she raised him to be a soldier, now she hopes to turn him into a politician. And Coriolanus, who cannot refuse her, once again submits, in the language of an obedient child: "Mother, I am going to the marketplace. / Chide me no more (III.ii.131-32)."
But, despite all her ambition and will, Volumnia cannot make him a politician; as a public figure, her son is a disaster waiting to happen, and the clever tribunes stand ready to exploit his first slip. Indeed, their skill is hardly tested: It only takes one accusation ("traitor") from Sicinius' lips to make him explode with a curse: "The fires i' th' lowest hell fold in the people! (III.iii.68)." Now the crowd speaks up, demanding Coriolanus's death, and the tribunes, victorious, can effect his exile while appearing magnanimous; with the mob calling for his execution, the punishment seems generous. Relishing his role, Brutus offers a patronizing explanation for the lighter sentence: "He hath / Served well for Rome (III.iii.82-83)." At this point, even Menenius stands helpless; Coriolanus has brought the whirlwind upon himself.