The Man Who Was Thursday: A Nightmare is a comic work that uses the trappings of a detective story to explore philosophical questions. Two poets, Gabriel Syme and Lucian Gregory, argue about anarchism at a tea party. To prove that he is a genuine anarchist, Gregory takes Syme to an anarchist meeting after receiving Syme’s pledge that he will not reveal what he learns to the police. As the meeting begins, Syme extracts a similar pledge from Gregory and then tells Gregory that he himself is a police detective.
The business of the meeting is to elect a member of the Central Anarchist Council, a group of seven, each of whom has one of the days of the week as a code name. Syme manages to take Gregory’s place on that body as Thursday. Bound by his word of honor not to call in the authorities, Syme feels as if he is alone against the forces of evil, who seem to be embodied by the leader of the Council, Sunday.
Syme endeavors to prevent the anarchists from carrying out a planned bombing, but as he chases those at the heart of the conspiracy through London and France, he discovers that the other council members are also police detectives, all appointed by the same mysterious official. When, as a group, they confront Sunday, he reveals that he is the chief detective as well as the chief anarchist, then leads them on a chase through London involving a fire engine, an elephant, and a balloon. The chase ends at Sunday’s home.
Bewildered, the detectives find themselves at a dress ball. They are provided with costumes representing the seven days of creation as described in the book of Genesis. Sunday explains that he has tried to give to the forces of order the dignity of which the anarchists boast—that of struggling alone against the world. Gregory, the only real anarchist, arrives and asks if Sunday himself has ever felt that awful loneliness. Sunday replies with the words of Christ: “Can ye drink of the cup that I drink of?” Syme then wakes from what turns out to have been a vision and finds himself back where the adventure began, talking with Gregory.