The novel opens on the eve of the First World War, immediately following the assassination of Archduke Francis Ferdinand in Sarajevo. In the United States, secret negotiations are underway concerning the Harkness shell, an important military device whose plans are stolen before delivery to the British Ambassador. Two British agents, Halkett and Gray, embark on a desperate mission to recover the stolen formula and escape to London while being relentlessly hunted by German agents across Europe. During his journey through France, Halkett meets James Warner, an American military painter residing in Saïs.
Warner agrees to carry a thin envelope containing the vital papers hidden in his breast pocket. In the provincial town of Ausone, Halkett encounters Philippa, a young cashier at the notorious Café de Biribi. Philippa is forced to act as an informant by her guardian, Wildresse, an ex-gambler and a double agent secretly selling secrets to the Germans while nominally working for the French government.
Philippa rebels against this life of espionage and flees to Saïs, seeking refuge and the friendship of Warner. As the threat of invasion grows and European nations mobilize for war, Wildresse and his thugs pursue Philippa to recover compromised documents. The climax builds through violent confrontations at the Inn of the Golden Peach, culminating in the formal declaration of war and mobilization. Amidst the chaos, loyalty, personal redemption, and a budding romance blossom between Warner and Philippa, while Halkett succeeds in securing the vital intelligence for his country.
