The Man Who Was Thursday is about a man named Gabriel Syme, who is a poet recruited by the London police into an undercover unit designed to combat intellectual threats to the government, specifically "anarchists," by which Chesterton means not quite what we would mean, but a wide-ranging group of intellectuals whose ideologies are closest to nihilism. Through Syme's cleverness, he successfully masquerades as an anarchist and is elected to the worldwide council of anarchists, of which there are seven, each named after a day of the week. Syme is Thursday, and the group is led by the intimidating and mysterious Sunday. If you think the fact that anarchists have an organization is funny, well, that's the sort of humor you're getting with this book.
Thing is, as the book develops, you find out one by one that all six of the "Sabbatarians," excluding Sunday, are policemen in the same unit tracking each other. I think that's a really funny twist, and it's certainly a funny book. When Syme's fellow anarchists explain why the former Thursday must be replaced, this is what they say:
As you know, his death was as self-denying as his life, for he died through his faith in a hygenic mixture of chalk and water as a substitute for milk, which beverage he regarded as barbaric, and involving cruelty to the cow.
These sorts of nihilistic and existentialist philosophies are the butt of Thursday's extended joke. Chesterton "claimed afterwards that he wrote this book as an unusual affirmation that goodness and right were at the heart of every aspect of the world," according to Wikipedia--at the heart of the book is an affirmation of the great order of God that rules over chaos. "Chaos is dull," Syme says, trying to argue with an anarchist and fellow poet that order and lawfulness are the most poetic things in the universe.
This book is only around 150 pages, but it's packed very tightly with a lot of ideas. Despite its philosophizing, its humor makes much of it a very light book, and some of the more "adventurous" scenes would make an awfully good film--there's even a car chase. But the book's final chapters, in which Syme seems to exit the reality of London and enter the fantastic and surreal world of Sunday's grand estate, and where the book's overt Christian themes are the most evident, that are the most impressive.
I would like to film this book, casting Cary Elwes as Gabriel Syme. If you are a movie producer, please e-mail me.