The Man Who Was Thursday is a philosophical novel by G.K. Chesterton that tells the story of Gabriel Syme, a detective who infiltrates a secret anarchist council led by a mysterious figure named Sunday. When Syme becomes the council's 'Thursday', he discovers that the other council members are also undercover police officers. Their attempted order is disrupted by a chaotic anarchist plot, leading to a series of surreal adventures that test both identity and reality itself. Ultimately, Syme realizes that all his adventures were part of a dream or fantasy he was having. The novel is considered a powerful influence on writers such as Franz Kafka and is widely considered his masterpiece.