The story is set in the area around Philadelphia in 1876, a few weeks prior to the opening of the Centennial Exposition. Vesper lives at Strafford Mansion outside Philadelphia with her guardians, Professor Brinton Garrett and his wife Mary. The household includes Smiler and Slider, twins met in earlier adventures, and Tobias "The Weed" Passavant, a young man of about twenty-four who is in residence studying the research of the late Dr.

Holly, Vesper's father.

The group is visited by President Grant who has a dilemma with which only Vesper can assist him. Children in care of the emperor of Brazil, who has come to Philadelphia for the opening of the centennial, have been kidnapped by Helvitius, and Helvitius has stipulated he will release the children only if the ransom is delivered in person by Vesper. Thereby begins the adventure to regain the children and outsmart Helvitius whose intent...