Paris, 1889. On the brink of economic recession and another civil war, France has managed to create stability by hosting the Exposition Universelle: a showcase of industrialized countries and their colonial possessions. After a successful opening, Paris begins to thrive—but the newfound prosperity is threatened by the arrival of an English woman with a mysterious illness. When the woman vanishes without a trace from her hotel room, and the hotel’s manager and doctor deny her very existence, her daughter enlists the help of a London reporter. As they scour the city for traces of the missing woman, they find a key witness in Katharine Eliot. They seek her help, unaware that Katharine is already taking pains to protect herself and her family from a murderous elitist, and that she has close ties to the people involved in the woman's disappearance. As the city goes to extremes to prevent a mass exodus of the fair, Katharine must decide whether to risk everything to achieve justice for a stranger—and how far she will go to stop a killer.