A famous Vermeer painting,
The Lady Writing, is stolen while in transit to a Chicago museum. Petra and Calder, classmates at the University of Chicago Lab School, think they may be able to solve the mystery. Petra has found, and then lost again, a related letter, and each of them may be getting supernatural messages about the theft, Petra directly from the Lady in the painting, and Calder from a set of pentominoes.
As they track down clues, their investigations lead them in many directions: an old lady in the neighborhood, a famous bookstore, their teacher, a book of freakish phenomena, a friend's disappearance, and a series of odd coincidences. No one is what they seem to be, and Petra and Calder don't know whom they can trust.