When The Man Who Was Thursday was first published, in 1907, the big terrorist threat came from anarchists, who threw bombs and assassinated people for a variety of complicated reasons. They turn up quite regularly in the literature of the time, quite often as vague caricatures representing some kind of destructive force or forces of evil. This is the case in The Man Who Was Thursday, where Chesterton uses the anarchists to represent all that is negative in the world, and doesn’t tie them in to any particular political movement.
The book’s an allegory, so no character can be taken at its face value–and to complicate matters, within the novel every character is revealed to be quite different from what he seems. (This is almost exclusively a male tale, born of a society where the men of the English ruling class were expected to move in men-only circles from an early age, through boarding school to clubs to Parliament.)
The plot is relatively straightforward: the poet Sykes infiltrates a group of anarchists nicknamed according to the days of the week (Sykes is Thursday). He is co-opted into the anti-anarchist police by a mysterious personage whom he meets in a completely dark room. The anarchists are led by a larger-than-life, terrifying character called Sunday.
The book has a repeating pattern of wild chases and moments of revelation that build on one another to become funnier as the plot thickens. For this is a comedy, although a subtle and disturbing one. I’d hate to spoil the book for you by explaining exactly what’s going on, but I can say that terror alternates with relief and a sense of the ridiculous. The climax of the book is quite thrilling and profound. The novel’s subtitle, A Nightmare, may give you a hint about the plot’s strange shifts and reverses.