O.Henry's short stories are quick and easy to read, so to compose a summary is almost redundant.
In a nutshell, Soapy is a homeless man in New York City at the beginning of the 20th century who needs to find a place to live for the winter. He is too proud for charity, and instead tries to get caught breaking the law so that he can spend the winter warm and fed in the local jail.
His attempts fail, and Soapy begins to think that perhaps it is time for him to straighten up his life. At that moment, he is arrested for loitering. The link below offers more detail.
O.Henry is famous for his effective use of situational irony to make a statement in a very short amount of time. Here the irony is that when Soapy wants to go to jail, he can't get arrested. When he decides that he doesn't want jail after all, he gets arrested for standing around, doing nothing.