Blanco Posnet stole the Sheriff’s horse, but as he was riding away from town he encountered a Strange Woman in the desert who was holding a child sick with the croup. She put her child in Blanco’s arms, and cursing his fate and his own weakness, Blanco gave her his stolen horse so she could ride into town and save the child.
Blanco sits to observe a magical rainbow that appears in the horizon and is captured by the posse chasing him. Before reaching town—the Rainbow Woman declares at Blanco’s trial—the child was “like lead in my arms.” Blanco laughs hideously: “Dead! The little Judas kid! The child I gave my life for!”
In spite of the death of the child, Blanco’s selfless action triggers a wondrous series of events: The Strange Woman refuses to reveal the identity of the man who gave her the horse, because “I thought God sent him to me.” The town prostitute (who had seen Blanco ride away on the stolen horse) changes her testimony and says that she never saw Blanco: “I only said it out of spite because he insulted me.” The Sheriff refuses to press charges because his horse was returned. To the chagrin of the hanging jury, Blanco goes free.
All this inspires Blanco Posnet to preach a moral sermon to the gathered townsfolk:
It was early days when [the Lord] made the croup…. It was the best He could think of then; but when it turned out wrong on His hands He made you and me to fight the croup for Him…. He made me because He had a job for me. He let me run loose til [sic] the job was ready; and then I had to come along and do it, hanging or no hanging. And I tell you it didn’t feel rotten…. I got the rotten feel off me for a minute of my life; and I’ll go through fire to get it off me again.
Blanco concludes that Feemy Evans (the town madam) is “a failure as a bad woman; and I’m a failure as a bad man.”
My theology and my explanation of the existence of evil is expressed roughly through Blanco. To me God does not yet exist; but there is a creative force constantly struggling to evolve an executive organ of godlike knowledge and power: that is, to achieve omnipotence and omniscience; and every man and woman born is a fresh attempt to achieve that object.
As to his methodology of treating serious subjects in comic fashion, Shaw argued:
Why should I not? Why should humour and laughter be excommunicated? Suppose the world were only one of God’s jokes, would you work any the less to make it a good joke instead of a bad one?
There are lessons to be learned from a reading or viewing of The Shewing-Up of Blanco Posnet. I take from it one overriding moral: we are rotten people, living in rotten towns, amidst rotten social systems. Since we cannot yet be Good, we try our best—with the help of the Holy Spirit reflected in Blanco Posnet’s “rainbow”—to be failures as bad men and women.
 Quoted in Henderson, Man of the Century, 589-590.