The Man Who Was Thursday

The Man Who Was Thursday is a novel by G. K. Chesterton, first published in 1908. Its central figure is Gabriel Syme, a policeman who works in secret, and the book follows him into an organisation whose members are named after the days of the week.

Syme meets an anarchist in a London park and uses the acquaintance to get himself admitted to the Central Anarchist Council, which is planning something that the police want to stop. He is voted into the seat called Thursday, which puts him inside the group and inside its secret, and he is still there when he discovers that one of the others is also working under cover.

From there the novel becomes a chase across Europe, with Syme no longer sure who is on which side of it and with his confidence in his own ability to outwit the others growing at the same time. At the end of the road is the council's leader, a man called Sunday, and the question of what he really is.