Coriolanus, the Roman general Caius Marcius has earned the name Coriolanus after many military successes. Inflated by these victories, he decides to pursue politics, though he is temperamentally unfit. He is encouraged by his mother, Volumnia, to pursue a higher rank and title through politics.
Act one begins in Rome at the end of the Tarquin dynasty. Riots have broken out after stores of grain for citizens have been withheld. The rioters blame Caius Marcius. Meninius Agrippa, an important member of the community from a rich household, tries to pacify the public but Marcius insists they don't deserve the grain since they didn't earn it with military service. Brutus and Sicinius, two of the three tribune leaders of Rome, discuss in private denouncing Marcius.
Marcius is unaware of their conversation and leaves to go fight the Volscian army, led by commander Tullus Aufidius. Aufidius and Marcius are mortal enemies who have fought many times, but Marcius still fights under the Roman army commander Cominius. Marcius takes an opportunity to raid the Volscian City of Corioli when Cominus is away meeting with Aufidius, and is able to capture the city for the Romans. He joins his commander and fights on, finally meeting and overthrowing Aufidius in a one on one battle. Aufidius is pulled away from the battle before losing his life.
Cominius awards Caius Marcius an official nickname of Coriolanus for his bravery in battle. This nickname increases his notoriety and power, and his mother encourages him to run for political counsel. His political career starts promisingly, with a show of support from the Roman Senate and some of the common people. But Brutus and Sicinius still plot to undo his aspirations start a movement to oppose him becoming a counsel. At this insult, Coriolanus becomes enraged and spouts off a verbal attack on the entire concept of rule in Rome. His powerful words are enough to cause Brutus and Sicinius to declare him a traitor and sentence him to banishment.
Coriolanus, exiled from Rome, finds his enemy Aufidius and tells him to kill him, since his own country Rome sent him away. Aufidius is honored that Coriolanus came to him, and along with his superiors, accept and welcome Coriolanus to their army. With their army backing him, Coriolanus plans a new attack on Rome.
Noble people of Rome like Cominius and Meninius try to convince Coriolanus not to attack the city, but both fail. Rome is panicked and Coriolanus' own mother Volumnia is sent to talk to her son, along with his wife Virgilia, their child, and a gentlewoman named Valeria.
His mother is finally successful in her attempt to stop her son from destroying his home city of Rome. Coriolanus has a change of heart and begins organizing a peace treaty between the Roman people and the Volscians. He draws the contract and goes to the Volscian capital to deliver it to Aufidius and his superiors. But upon his return he finds that Aufidius has organized a group of conspirators that end up killing Coriolanus for his betrayal to their people.