The book describes a Europe under threat from terrorists, from anarchists, dynamiters and assassins. To meet the threat, London’s Metropolitan Police have formed an elite anti-anarchist squad, tasked to infiltrate the enemy. Following up a chance conversation, undercover detective Gabriel Syme attends a meeting of the General Council of the Anarchists of Europe, and is even elected to a vacancy in their leadership. This tight group has seven leaders, each of whom takes his codename from a day of the week. The mysterious overall boss is Sunday. Syme himself becomes Thursday.

Thirty years ago, a British newspaper took an unscientific survey of current and former intelligence agents, asking them which fictional work best captured the realities of their profession. Would it be John Le Carré, Ian Fleming, Robert Ludlum? To the amazement of most readers, the book that won easily was G.K. Chesterton’s The Man Who Was Thursday, published in 1908.

This was so surprising because of the book’s early date, but also its powerful mystical and Christian content: Chesterton subtitled it “a nightmare.” But perhaps the choice was not so startling. Looking at the problems Western intelligence agencies confront fighting terrorism today, Chesterton’s fantasy looks more relevant than ever, and more like a practical how-to guide.

Inevitably, my description has to give some spoilers, but they do not reveal any of the book’s core revelations.

The book describes a Europe under threat from terrorists, from anarchists, dynamiters and assassins. To meet the threat, London’s Metropolitan Police have formed an elite anti-anarchist squad, tasked to infiltrate the enemy. Following up a chance conversation, undercover detective Gabriel Syme attends a meeting of the General Council of the Anarchists of Europe, and is even elected to a vacancy in their leadership. This tight group has seven leaders, each of whom takes his codename from a day of the week. The mysterious overall boss is Sunday. Syme himself becomes Thursday.

Syme seems to have pulled off a major coup in the war against terror. But matters become more complicated, when he finds that he is not the only police infiltrator in the anarchist leadership. He discovers first another police spy, then another… could all the anarchist leaders really be detectives, or are all the detectives really anarchists? And who on earth is Sunday? Or is he not of this earth? As the detectives hunt the terrorists, and are themselves hunted, Chesterton creates a masterpiece of paranoia.

Chesterton himself was a bookish man with no real-world experience of police or intelligence. He did however have the dubious blessing of living in the first golden age of European terrorism, when issues of revolutionary subversion and counter-terrorism regularly filled the headlines. Since the 1880s, anarchist and revolutionary movements had carried out many violent acts across Europe and especially Russia, assassinating public figures and bombing trains and public meeting places. Scarcely less dangerous were nationalist and anti-imperial militants. Britain itself faced continuing threats from Irish and Indian activists.

Then as now, governments realized that the only way to defeat revolutionary terror was to penetrate and infiltrate the active groups, to gain intelligence about forthcoming attacks. Some agencies, though, carried this strategy to baroque extremes.

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