The play begins in the city of Rome, where the common people, or plebeians, are rioting against their rulers, the patrician class, whom they accuse of hoarding grain while the common people starve. The plebeians demand the right to set the price of grain, rather than accept a price imposed by the Senate (the governing body, run by the patricians), and they single out Caius Martius, a patrician general and war hero, as the "chief enemy to the people"(I.i.7-8). As they make their way to the Capitol, they are intercepted by Menenius, a patrician and a friend of Martius, who tells the mob that the patricians have their best interests at heart. He compares the role of the Senate in Rome to the role of the stomach in the human body: The stomach serves as a storehouse and collecting-place for all the nutrients and then dispenses them throughout the rest of the body; similarly, the patricians collect and dispense grain to the entire city.
As Menenius and the rioters argue, Caius Martius himself comes in, and delivers a general curse to the mob, calling them dogs and cowards. He then tells Menenius that the Senate has agreed to allow the plebeians to elect five "tribunes," or representatives, to advocate for their interests in the Roman state. At that moment, a messenger dashes in, bringing word that the Volsces, one of Rome's enemies among the Italian tribes, are arming for war. Martius declares that the war will be good for their city and notes that the Volsces are led by a great general, Tullus Aufidius, whom he respects as a worthy adversary. A group of Senators has come in, and they now order Cominius (who is the consul, or chief magistrate of Rome for the year) and Titus Lartius (another patrician) to command the impending war--Martius will act as a lieutenant under Cominius. The crowd disperses, and the Senators return to the Capitol to prepare for the campaign.
Meanwhile, the plebeians have already elected their tribunes. Two of these, Sicinius and Brutus, have been watching Martius's behavior, and now they both comment on how proud and domineering he is. Sicinius wonders how he will bear being under the command of Cominius, but Brutus points out that by being second-in-command, Martius will escape blame if things go badly, yet he will receive all the credit if things go well.
The mob of plebeians, which holds the stage as the play opens, lacks an individual identity but nevertheless constitutes one of the most important "characters" in the story. These commoners form something of a rabble, open to manipulation by the play's politicians, but Shakespeare does not portray them in an entirely negative light. They have taken up arms, true, but not without cause: As one of them puts it, "the gods know I speak this in hunger for bread, not thirst for revenge (I.i.22-23)." Moreover, their principal complaint in this scene seems altogether reasonable: Why should the patricians control the supply of grain in a time of famine, one wonders--and indeed, the eloquent aristocrats never sufficiently answer the question.
Of course, Menenius does makes an attempt at a response, with his story about the stomach and the body. His behavior toward the plebeians contrasts starkly with Martius's--the common people like him, calling him "one that hath always loved the people"; they say of him, "...he's one honest enough! Would all the rest were so!"(I.i.49-52). Although he does not genuinely care for them any more than Martius does (he never actually takes their side in any of the play's political disputes), the people nevertheless favor him because he possesses a gift the play's hero lacks--the gift of public relations. In this scene, he takes an angry mob and quiets it with a story. "You must not think to fob off our disgrace with a tale," one of the plebeians says, but that is exactly what Menenius does. His deftly politicking speeches contrast sharply with Martius's language here, which is primarily constituted of sputtering curses: "what would you have," Martius asks the crowd, "you curs / That like nor peace nor war (I.i.166-67)?" The pattern for the play is set: While Martius's bullheaded pride and brashness may serve him well on the battlefield, his lack of delicacy will prove his undoing among the populace.
Menenius's little tale does more than highlight the contrast between his persona and that of Martius: It also offers a kind of rudimentary political philosophy for the Roman body politic, which has only recently expelled its last king, Tarquin, and made itself a republic. The play shows us a city suffering from a power vacuum; wily patricians like Menenius and crafty demagogues like the tribunes now struggle to fill this vacuum, Menenius with his organic conception of the state and the tribunes with their notion of popular rule. Moreover, this political situation can be traced back to Martius; we learn that as a youth he had a hand in King Tarquin's overthrow. One can, thus, see the play's initial situation as an Oedipal moment: The young Martius has overthrown the royal father-figure and is poised to take his place--except that in republican Rome, the kingly Martius cannot take Tarquin's place without becoming himself a tyrant.