The Man Who Was Thursday: A Nightmare is G.K. Chesterton's surreal masterpiece—a philosophical thriller disguised as a detective story. The novel follows Gabriel Syme, a poet-turned-detective who infiltrates a secret council of anarchists, only to discover that nothing in this shadowy world is what it seems. Each member of the council carries the name of a day of the week, and all of them are masks behind masks, shifting identities, and symbols of a deeper, more uncanny truth. As Syme chases the elusive figure known as Sunday, the story dissolves into a dreamlike pursuit filled with paradoxes, theatrical confrontations, and moments of haunting beauty. Chesterton explores themes of free will, fear, disguise, divine mystery, and the strange borders between comedy and terror. Written in 1908, the novel feels eerily modern: it examines the anxiety of political chaos, the fragility of identity, and the human desire for meaning in an absurd universe. Part satire, part allegory, part nightmare, this is a book that cracks open the world and asks the reader to look directly into its shimmering uncertainty.