The Man Who Was Thursday was written by G. K. Chesterton and follows newly recruited Scotland Yard detective Gabriel Syme as he infiltrates the dangerous underworld of the European anarchist council. Syme is a member of a special antianarchist division of the police and finds his way into the secret group through a poet he befriends, named Lucian Gregory. Once Syme is on the inside, he discovers the council is made up of seven men, each named after a day of the week, and is led by the mysterious president known as Sunday.

He discovers the group's intention to kill a czar who will soon be visiting Paris, but in his attempts to stop this plot he soon realizes that all the other members of the anarchist council are also undercover police tasked with bringing the group to justice. With all their identities now in the open, they join together to find out the real truth behind why Sunday set them all against each other and who the enigmatic leader really is. After chasing the president through the streets of London, they eventually confront him, and Syme's nightmare comes to a surreal end.

The book was first published in 1908 and has been adapted many times over the years, including two adaptations for BBC radio as well as an abridged radio play adaptation written by Orson Welles.