Mrs. Basil E. Frankweiler, a wealthy widow who collects various files and artifacts, writes a letter to her lawyer, Saxonberg, complaining that his most recent visit was boring since he had too much on his mind. She explains that she wants to make changes to her will. She includes the following text, which is a copy of some files in her records, as explanation.

In the mid-1960s, 12-year-old Claudia Kincaid lives in Greenwich, Connecticut, with her parents and three younger brothers. Feeling underappreciated at home compared to her spoiled brothers, Claudia plans to run away from home and stay in the Metropolitan Museum of Art. Mrs. Frankweiler speculates that Claudia may also be bored with her mundane, repetitive life. Claudia plans to take Jamie, her second-youngest brother, with her since he has a lot of money saved up. For a few weeks, Claudia saves her weekly allowance to cover train fare; she also manages to recover a spare train pass from her parents’ trash.

Claudia waits until the Monday before the Wednesday she plans to leave to tell Jamie. On the way to school, she sits by Jamie on the bus.