November 1916.

The war of attrition is taking its toll on the Allied powers and cracks are starting to appear.

Captain ‘Blinker’ Hall, Head of Naval Intelligence, must strive to keep the alliance with France alive – and use all his guile to bring the Americans into the war.

Johnny Swift, a reckless former diplomat turned soldier, is convalescing in London, working for Naval Intelligence.

Hall knows how to use Swift’s talents for duplicity to their fullest and sends him to Paris to flush out a traitor undermining the French war effort.

Room 40, the Royal Navy’s code breaking unit, deciphers a telegram that presents Hall with a dilemma. Its use could recruit America to their cause – but also give away the secret that the codes have been broken and cost Britain the war.

Swift takes up his role in Paris and is soon caught in a web of intrigue involving Mata Hari, the Dreyfus Affair, and the catastrophe of the Nivelle Offensive.

The fate of the conflict rests on a knife edge.

The traitor still lurks in the shadows of Paris’s Art Nouveau grandeur and Swift must locate him before he can betray the Allied cause.