The Secret of Jules and Josephine

In Emeraldia (Ireland), no one usually sees the fairies, but everyone knows where they live - in the green mounds that are scattered all over the countryside. The fairies protect Emeraldia from the evil that lurks beneath - the Black Air of the Underworld - but they depend on humans to respect and protect their mounds. A catastrophe happened in 1927 Paris when Finn O'Hara, the man who protected the most important mound, was forced out of Emeraldia and murdered in Paris. Then other fairy habitations were dug up, most of Emeraldia's fairies were made homeless, and so much evil fills the air that good humans are fleeing the country. The fairies know that their only hope is for history to be re-written. They have found a portal back to 1927 Paris, and they have asked Finn's great-grand-niece, 8-year-old Cornelia to go through it (accompanied by the exotic, dance-mad fairy, Tansy) and warn him of the danger he is in. To save Finn, Cornelia and Tansy stalk criminals on the Blue Train to Monte Carlo, on the Aquitania to New York, and on board Lindbergh's plane.