With the Entente Cordiale still in its infancy, the Sûreté had assigned Inspector Gautier to keep a protective eye on English visitors to Paris for the racing at Longchamps. But it was not the English who engaged Gautier's attention, but an Irish surgeon - Michael Breen.

Breen, fêted by the ladies of Paris, is accused of a trivial assault - and almost at once Princess Hélène's daughter goes missing and a shop assistant at Au Bon Marché is found dead just as Breen flees the country. Inspector Gautier is hot on his heels to Dublin, whilst a third murder back in Paris complicates the affair for all involved.