The story revolves around the historical figure of Giovanni Colombini, a rich merchant and landowner from Siena, who underwent a religious conversion, dressed himself in sackcloth, and wandered from one castle to another preaching humility and charity towards the poor.
He spends an important period of his life in Monticchiello, where his revolutionary doctrines lead to his being imprisoned by Count Cione, a local feudal lord.
But the local people, persuaded by the fact that Colombini has donated all his lands to the peasants, rebel agains the Count and demand justice.
Colombini urges everyone to show forgiveness, and leaves Monticchiello with a band of new followers.
Whereas The Heroine of Monticchiello presented the notion of a struggle for political freedom, this play concentrated on themes of social justice.