Hussey's assignments and dialogue even came from Balliett's classroom. She chose the setting of Hyde Park, Chicago, where she currently lives, because she considered it full of secrets that children could discover. Plot summary The story begins with three residents of Hyde Park receiving anonymous letters. The letters ask for the recipients’ help in solving a centuries-old art mystery. In the days that follow, Petra and Calder’s teacher, Ms. Isabel Hussey, gives several assignments related to unique letters and letters found in works of art. One day after school, Calder follows Petra to Powell’s Bookstore, and the pair run into each other, beginning their unusual friendship. Calder is obsessed with pentominoes that he keeps in his pocket and uses to send and receive coded messages, while Petra is an adventurer. Ms. Hussey’s next assignment requires the children to present their interpretation of art. Calder chooses a Geographer’s box with a painting on it. Petra chooses Lo! by Charles Fort, a strange book in which Fort posits that life is not a series of coincidences but is an interconnecting web of patterns. Calder and Petra learn that Lo! used to belong to Mrs. Louise Coffin Sharpe. Calder visits Mrs. Sharpe and notices she has a copy of the picture from his Geography box, The Geographer by Johannes Vermeer. Meanwhile, Petra has a vision of a lady in an antiquated dress with pearl earrings. For Halloween, she dresses as the lady, and Calder recognizes her as the woman in Vermeer’s painting, A Lady Writing. A Lady Writing is on its way from the National Gallery to an exhibit in Chicago. Before the painting arrives, it’s stolen. The thief sends a letter to the Chicago Tribune stating that he’s stolen the painting to raise awareness that someone else painted some of Vermeer’s paintings. The thief claims that once the art world has repudiated the authenticity of the paintings, he’ll return A Lady Writing. The children learn that Ms. Hussey received one of the letters and begin to suspect that the painting is somewhere on school grounds. Calder has an epiphany, connecting himself and Petra with the number 12. This clue leads them to the painting. On exiting the school, the thief begins chasing them, and Calder stays behind, urging Petra to run on with the painting. Petra and a policeman return to the playground where she last saw Calder. While they’re searching, the