The story begins when Gabriel Syme, a poet and member of a special group of philosophical policemen, attends a secret meeting of anarchists, whose leaders are named for the days of the week, and all of whom are sworn to destroy the world. Their chief is the mysterious Sunday - huge, boisterous, full of vitality, a wild personage who may be a Chestertonian vision of God or nature or both. When Syme, actually an undercover detective, is unexpectedly elected to fill a vacancy on the anarchists' Central Council, the plot takes the first of many surprising twists and turns.

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In an article published the day before his death, G.K. Chesterton called The Man Who Was Thursday "a very melodramatic sort of moonshine." Set in a phantasmagoric London where policemen are poets and anarchists camouflage themselves as, well, anarchists, his 1907 novel offers up one highly colored enigma after another. If that weren't enough, the author also throws in an elephant chase and a hot-air-balloon pursuit in which the pursuers suffer from "the persistent refusal of the balloon to follow the roads, and the still more persistent refusal of the cabmen to follow the balloon."

But Chesterton is also concerned with more serious questions of honor and truth (and less serious ones, perhaps, of duels and dualism). Our hero is Gabriel Syme, a policeman who cannot reveal that his fellow poet Lucian Gregory is an anarchist. In Chesterton's agile, antic hands, Syme is the virtual embodiment of paradox:

He came of a family of cranks, in which all the oldest people had all the newest notions. One of his uncles always walked about without a hat, and another had made an unsuccessful attempt to walk about with a hat and nothing else. His father cultivated art and self-realization; his mother went in for simplicity and hygiene. Hence the child, during his tenderer years, was wholly unacquainted with any drink between the extremes of absinthe and cocoa, of both of which he had a healthy dislike.... Being surrounded with every conceivable kind of revolt from infancy, Gabriel had to revolt into something, so he revolted into the only thing left--sanity.